Ok, so there are a number of reasons that would lead someone to want to trim their dreadlocks, the main obvious one is length, not every one wants knee length dreadlocks. The longer your dreadlocks are, the more time you will need to spend washing and drying them - more dreadlock = more dreadlock to wash.
Connected to this is weight, the longer your dreads are, the heavier they will be, especially when wet. Heavy dreadlocks can be uncomfortable and can lead to neck pain and head aches as you have something pulling at your head 24/7. The longer they get, the heavier they get and therefore the harder they pull on your head. Also if they do get heavy enough there is the possibility that they can weaken the root and break off.
You will find, especially with backcombing or with crocheted dreads that the original dread, so by that I mean the dreadlock that gets started from your normal straight hair, this original dreadlock can have a different texture to the new growth. Backcombed and crocheted dreads have forced knots in them, where you've forced hair into a certain position where it might not have ordinarily gone resulting in stiff, tough dreads, with my own I thought that they resembled club-like lumps of hair, whereas the new growth was like soft rope. The original length can also be very lumpy and bumpy from shrinkage whereas the new growth should be much more uniform and so many people like to (once they have enough length,) trim off the original length to just leave the new growth which is usually softer, bendier and less lumpy.
If you're unhappy with the tips of your dreads - if they're long and loose and you want them more short and blunted but don't want to crochet them up, you can trim off the loose length.
If you're congoing some dreadlocks together you can often find that they'll merge at the roots and part way along the body of the dread, but leave the matured tips separate and so I like to trim those off.
If you've waxed your dreads in the past and want to get rid of it the only 100% certain way is to remove the waxy part. I have been through this myself where the waxed section, no matter how hard I tried would always remain stiff and...waxy. I was infinitely happier with my hair once I had trimmed off all of the original length.
How to trimFirst off I'm not going to recommend trimming for really young dreads. If you're trying to trim the dreadlocks for aesthetic reasons but the dreadlocks haven't fully matured yet I would definitely recommend waiting for reasons I'll list in the Pros and Cons section.
Trimming itself is pretty easy, you simply take some strong scissors and snip across the dreadlock. It's always better to take off a little at a time because obviously you cannot undo a cut. If you're cutting off original length or waxy length it's pretty easy to see where to cut because you can probably tell where the new growth begins and just cut there to remove the original length and leave the new growth. But if you're cutting just to keep them at a manageable length it can be a daunting task to just hack away at them. What I do is I decide how long I want them to be, which is normally around shoulder length. My ideal length is where the dreads are just long enough to be comfortably tied back. So I will trim one dread to a length that I like and then take note of how much I removed. So for example if I trim one dread and remove 4 inches I will then go around all the other dreads and remove the same amount from each, that way I keep the character of the dreadlocks and the overall look, but just have them all slightly shorter.
After you've trimmed I highly recommend doing a deep cleanse soak or at the very least a good wash. There will be lots of little completely loose hairs ready to fall out and so giving them a soak and a rinse will wash them out straight away rather than have them shedding throughout the day. Also freshly trimmed dreads will look freshly trimmed, the tips will look all squared off and giving them a wash will help loosen the tips and make them look more rounded and natural again.
Pros and cons of trimming dreadlocksPros:
The dreadlocks will be lighter, in some cases MUCH lighter. When I trim my dreads my head feels weightless afterwards. Lighter dreads will give relief to your head, neck and upper back!
The dreads will be quicker to wash because you don't have as much hair to wash, but more noticeably they will be quicker to dry. When you cut the tips off your going to be leaving the tips soft and open and so water will rinse out of them much more easily. Also reduced length means that the water doesn't have to be squeezed so far to get out of the dread. Oh, and obviously less dread means less dread to dry.
Removes wax and damaged dread length.
Removes original hard or lumpy dread length
and any other aesthetic pros you care to think of!
It's messy, you'll get little loose hairs falling out everywhere so it's best to do it outside. When you wash the dreads you'll have to be careful that you don't let all these loose hairs wash out and clog your drain!
If you cut too much you can't get the length back... unless you sew the dread back on!
If you trim young dreads / dreads that aren't yet matured the tips can fray! this is why I don't recommend trimming for young dreads, they aren't always strong enough to cope with it yet and so the new tip can fray out and cause the whole dread to fall apart. Luckily this is only an extreme worst case scenario, but I definitely recommend waiting as long as you can because more dreadlocks problems will sort themselves out if you give them enough time.