Aluminum can be bent a little, but not a lot... trying to get 1/4" bend in a 1 foot or larger span should be no problem, just don't heat the aluminum to get teh bend because these alloys are also heat treated in order to keep their hardness and strength, it's this same hardness that makes bending the aluminum a bit on the risky side. I'd say no more than about 15 to 20 degrees for a bend and keep the bend radius as loose as possible.
Most aluminum bike frames are made from 6061 T6 for it's strength and corrosion resistance, 6061 can also be welded on but needs to be re heat treated again to bring back the hardness and strength.
If you need to bend out the rear forks to fit a wider tire or for sprocket clearance, it can be done but do it a little at a time to prevent cracking, and also, you don't want to stress the aluminum by drawing it in with the bolts such as widening the rear for more clearance but going wider than needed then using the wheel nuts to bring it back in during assembly, you can use washers to take up this space or bend it back just a little closer so it's not under any stress after things are assembled etc.
One nice tool for bending rear sections is an exhaust manifold spreader for a small block Chevy... like this one... you can sometimes find these at auto parts stores like O'Reily's or Pep Boys etc... http://www.ebay.com/itm/Lisle-13000-...item35d960636f
or you can order one from ebay or Amazon if needed, but these will do most frame bends when you need to spread something out since these are nice and controlable.