My understanding is, which may be wrong, is that as long as you use a UL listed product that is equivalent to the insulation on the wire that you are fine. i.e. 90 degree rated heat shrink for 90 degree wire.
Heat shrink is pretty quick to do. I have a little lighter that is made for heat shrink. I will post a link if I can find the page for it.
I need to get one, I just use a soldering iron or a lighter.