Finn & Spin,
I concede all that you say about rockered skis skiing shorter; I know all that.
I just personally wouldn't want a high 180's or more ski when things start getting tight, but that's just me. I find just a few extra cm's does start to feel a little unwieldy in tight spots, and I'm an advanced skier who ski trees routinely. I may not be as advanced as either of you, but I've been Hack-skiing my way through 'em for several years now. So, if some of us are more comfy on 180ish skis in the trees in powder, than I would think a newbie tree-skier of roughly the same size likely would be also.
The OP says he wants to "get into" skiing trees next season and is looking for a 170ish length. I agree that's too short, but I also think it means he might not be comfortable with a 15-20cm leap in length for his first foray into the tighter confines of eastern trees. Wouldn't you guys agree that as a "starter" tree ski, he might find the 180ish length a little easier to manage and more confidence-inspiring?
OP, if possible, you should demo different lengths in addition to different skis. 170 is too short for sure, but a 180 is not too short for your purposes.
A 185-190 may or may not be too long to start. Unlike the short & long extremes, I just don't think you could wrong with a 180ish length at this point, given your goals.