You can usually go +/- 2 cm from optimal and still get a reasonably good fit. I find it's usually easier to make a slightly undersized frame fit than slightly oversized. Amount of seatpost showing is a "don't care" (unless you're trying to achieve a certain aesthetic). Stem length is where you don't want things to get too crazy. As long as you can keep the stem in the 80 - 120 mm range, you shouldn't have any issues. Longer/shorter than that does start to make the steering feel a little off.