This question was posed to me by a potential client who's hair has lost it's natural curl pattern due to heat damage. Her hair is completely natural but bone straight. As the new growth grows in, it becomes obvious that the hair has been damaged because of the two totally different textures and no there is not any relaxer on the ends.
When you have heat damage or product damage that may have changed your natural curl pattern, it is not necessary to cut the damage hair before your install with Sisterlocks. However, the maintenance will be more labor intensive because you will need to set your hair often. If you don't set your hair, your new set of Sisterlocks will just hang straight. Most new Sisterlocks have curly tips due the one's natural curl pattern but when no natural curl pattern exist, you will need to style a little more.
I say no big deal at all. It is a personal preference. I just want to put it out there for those natural beauties that like their hair straight all the time.....DAMAGING!!!! Be careful and give your hair a break from time to time.