Along with Kitty's notes, this should help some.

http://smanerdherd.com/forum/index.php/topic,499.0.html

One point I would argue is that the heat of even a hundred resistors is going to be so relatively low,relative to the density of most any plastic model, as to be negligible.  Further, if you are wrapping the resistor in heat shrink or electrical tape as you should, this will also diffuse the heat, so no hot spots should exist.  And there is little if any chance heat levels could ever get so high as to warp modern model plastic.  Granted some smaller, older kits could be fragile.  Maybe if you run your hundred leds for a week constantly, sure, the model will be very warm to the touch.  But if you're doing that, you already have some probs.  ;D

But resistor heat and led heat is really not a genuine concern for anyone running their lights for as much as even a few hours a day.  And how often are you really gonna do that anyway?  You turn it on, show your friends, marvel at your handy work for awhile now and then and you turn it off.  Right?

Absolute worst case: a resistor laying directly against the interior plastic melts a resistor-sized spot all the way through. Solution: You call it battle damage or get some putty, you get some paint, you prove your modeling mettle.