Although you may be a bit leery of knots slipping on you, be careful not to pull too hard when you snug them up (before cutting off the extra). That's a great way to snap the string and spoil the whole job right when you're doing the finishing touches. Yes, as a matter of fact, I have done this... but not more than twice.
When in doubt leave just a little extra I say. Every now and then you'll deal with a crazy-slippery string that might be more prone to slipping, even with a proper knot. Spiraltek from Babolat is a current string that seems insanely slick. The last time I installed some of this stuff for a pal's client, I didn't trust my knots, so I put a tiny bit of clear nail polish on them for peace of mind.
Even if you leave a little extra length when you snip the knot, it's not a big deal to trim that tail slightly shorter (without overdoing it) using perhaps a nail clipper. I occasionally like using a nail clipper for when I've cut the extra after knotting some poly, but I've left a sharp point or barb sticking out too much. A nail clipper is great or taking off that tiny corner and blunting the end of that string without taking it right down to the knot itself.