B-Tension screw
 

If I adjust the b-tension screw past the standard screw limits is there anything I have to watch out for? How far could I go? This is on a Shimano Deore rear derailleur.

What do you mean 'past the standard screw limits' and why would you want to do that? Is this a custom project of some kind? If all of your drive-train components are stock, there should be no need to adjust it past its normal limits.

See about 1:10 in this video tutorial:
http://bicycletutor.com/adjust-rear-derailleur/

When I say "standard screw" I mean the b-tension screw that comes standard with the rear derailleur (4x12).

Yes this is a custom project. I was having problems with the Jockey pulley rubbing the 34 in the rear. I made sure the chain length was right but that only solved the large to large issue. When I move the b-tension back it helps but it needs to go further than the screw allows. I tried the reverse screw trick and again that helped some. I was thinking about changing the screw to a 4x20.

I just want to understand if there is something I have to watch out for. I don't want to overstress the spring.

The B screw can be ineffective due to the configuration of the drop-outs on some frames. As the screw has to push against a small platform on the drop-out it sometimes happens that it cannot gain proper purchase. I've replaced a few with longer Allen-headed bolts instead of a screw and I've also built up the platform on the drop-out by brazing. I have never had a problem with the tension spring on the derailleur but others might know better.