Full length is better. They tend to want to curve the comb a bit on the ends and a guide all the way will help.
I'm facing this problem now. I have two populated TBH's. On one (hive #5) the bees naturally attached themselves and hung right in the middle of the hive in the middle of the bars. This hive is doing great. My first inspection was today and it was pretty easy. However, my 2nd top bar hive (#6) the bees congregated from the start to the front left side of the hive. My starter strips (1/4"x1/4" square tacked on wood) are only 9" centered on the 17" top bar. I build this according to the plans I built from (§¤«£¿æ book). Hive #5 is built the same way, but it started in the center of the bars so it's OK. Hive #6 starting on one side, that side's comb is all messed up. It's built across multiple bars then when it finally hits the 9" starter strip, they straighten it out. However, during my first inspection today, I was not prepared for this and wound up damaging the comb (nothing fell, but it did break where they built across the top bar boundaries).
My question is now what? They are a 2# package that was installed 1 1/2 weeks ago. I really hate to set them back by messing with their comb however I do not want to deal with this for the rest of the hives existance! Any thoughts on what I can do to solve the problem w/o setting the bees back too far?