In reply to huckleberry:
I wouldn't be super worried about foot swell unless that's something you've noticed you suffer from a lot. Take a thick pair of socks to try them on with too!
If you're going to be doing a lot of front pointing in crampons (and it sounds like you might be!) make sure there's enough space at the toe such that you're not going to bash your toes off the front of the boot every time you kick - finding something solid and giving it a few good kicks to see if you're hitting the front is ideal ... but the shop might be a bit upset ...
Other than that, normal boot fitting rules apply!
Break in time will obviously vary with each boot, but a couple of days out on the hill is usually enough. Modern boots seem to require less breaking in than traditional ones, but this is a point of endless debate!
Post edited at 01:56