I'd say trimming is necessary. In all planes you can easily take off with trim in neutral position, but it considerably lessens your workload if you trim while flying.
The Mustang, well with that thing trimming is very important, since any jerks and quirks throw off your aim when you're zooming in on the target at 600 km/h IAS. You should have a Mustang session with either me or Schröder so that we could show you some of the basics of that aircraft. It is quite different than for example 109(which is quite hard to stall, unlike the Mustang, thanks to the slats). Watching these is a good way to start:
http://352ndfg.com/smf/index.php?topic=1214.0