Well, I've read people say that latex doesn't stretch, and therefore stretching it isn't useful, but I have put my neck seals over a 7" tank overnight before I cut them. The latex does seem to relax a bit over the first several weeks, so you don't want to cut the seal until it's perfect, but leave it just a tiny bit uncomfortable to begin with.
Cutting seals is not that difficult. It's best to have an assistant to hold the seal under slight tension, and you need a very sharp cutting instrument. You want as continuous a cut as you can manage, because any abrupt edges will be starting points for later tears. If you are at all unsure about your ability to do it, take it to a shop and pay them. Messing it up can ruin the seal immediately by making it too loose, or shorten the life of the seal significantly.