they look like they are doing pretty good. I;m pretty interested in this and I have a buddy that has been trying to grow in a window sill too.

Anyways... about the stretch thing, yep they can stretch even up to 4x their height from veg... that would probably only happen with a pure sativa though.

as for tieing them down - yeah that is basically it. Obviously caution must be used, normally people start doing it really early on I believe before the stems harden and get difficult to bend... even if you do snap a stem its probably not the end of the world... generally you can just straight it out, tape a splint around it and it'll heal in a couple days but obviously you want to avoid that if you can.

I'd suggest you experiment on the weakest of the bunch first to get the hang of it.

I think if you look around in the advanced techniques form for "LST" (Low Stress Training) you'll find some more info about it.