In this video I talk about an incident that drove me to almost remove my dreadlocks.
Provided at least a somewhat active lifestyle you should be able to build strength in your neck/shoulders at a rate that will easily cope with the steady weight increase of the dreadlocks over time, however in situations where the weight changes rapidly (as is mentioned in the video) - OR situations where your neck/shoulder muscles become injured, your ability to deal with the weight may become seriously impaired.
Two to three years into my dreadlocks I was involved in a car accident that left me with a minor neck injury - and despite it only being a minor incident I ended up in a pretty bad situation with regards to my hair. The soreness in my neck was greatly amplified by the weight of the dreadlocks and this inhibited my ability to effectively recover - while drying my hair at the time I had to lay down as I was unable to effectively carry the weight of the hair. I ended up wearing the neck brace most of the time to support the weight on my head - and this lead to weakening of my neck muscles, which pretty much made the whole thing worse. Over time I was gradually able to recover, but it took several months - and eventually I trimmed some of the length of my hair to help my neck to recover faster.
I can say that the incident there was the only time that I've had serious issues with regards to the weight on my neck, and in that situation the dreadlocks were not directly responsible. My hair has since grown out again to a length much longer than it was at the time, but as I have not again suffered any injury to my neck, I have been able to adjust and strengthen at a rate which has been able to cope with the increasing weight of the hair.
You cannot really "prepare" for such an incident, as it's something that everyone would rather avoid at all costs - however I thought it was worth sharing!