Asking the author for permission is of course advisable, and you should definitely go for it. It will make everyone happier if people are agreeable on all sides.
However, don't get too tied up in it. First of all, I consider all scripts posted in OFPEC or elsewhere to be entirely "open source" : I can take whatever I want from them, edit it to suit my needs, and repost them or use them in my missions/addons/whatever. Yes: out of common courtesy I'll make sure to always add credits where needed, but asking for permission is hardly necessary. This is a shared repertoire of knowledge we have here, and the more people who pick apart stuff to make something new/improved/just different, the better.
As to addons, the question becomes a little more complicated. To get to the scripts, you need to open up the addon and extract them. However, in a large majority of cases this won't do you any good: most scripts refer directly to the addon itself (such as the one-shot RPG in the RHS_VDV/Marines addon), incorporating eventhandlers or classes that you might not be using. I see nothing wrong with opening up such a script and copying and getting inspired by parts of it; my math skills suck and without Mandoble's stuff I'd be lost half of the time. Same goes for all kinds of particle effects: I steal happily here as well, from missions, scripts, wherever. There can be no intellectual copyright on a couple of math-commands strung together somewhere, or someone's particlearray (IMHO).
But as to whole scripts more or less adapted out-of-the-box...well...I think the same goes really as for addons: ask the creator nicely, and if he doesn't respond/can't be reached/has gone AWOL, take a leap of good faith and go forward with it anyway. You will probably have to adapt the scripts for your usage so much anyway that they'll be nigh unrecognizable.
Long rant over.
Wolfrug out.