<VV> machining info

[ScottyGrover at aol.com]

I've done some digging on the 'Net for pricing on aircraft-grade  
aluminum--can anyone tell me--if I have a part that's to be  machined to  a maximum 
4.5" x 6.5" in cross-section, just how much  should I allow over those 
dimensions when I go to purchase the metal? I don't  want to go excessively 
oversize--that 6061 is g**-awful expensive.  
 
Scotty from Hollyweird