I've been turning this question over in my head and I decided to post it here and see what you guys think...
My hair just recently reached my shoulders on the sides. It hangs two or three inches further down in the back because, of course, it's starting from further down on my head.
I've been thinking that I probably won't want the hair to hang below my shoulder blades (I guess - we'll see when it gets there), so at some point I'd want to have it trimmed to one level across the back.
Now that it's at my shoulders, I think it might look neater if I had the back trimmed now. On the other hand, I really hate the thought of losing 4-6 months' growth back there! If I wait until the sides are longer and hang down behind my back, I can get the back trimmed without noticing it so much.
Of course, the question may be moot for now, since I don't know where to go for a trim. The last stylist I went to tended to yank the comb through my hair and snag it a lot, and I hate to think what that'd do to it now. My partner's afraid to trim it for me for fear of messing it up. So... I guess it'll have to wait until I find a stylist who's neither scissor-happy nor comb-happy.