Not sure what you mean by "angled outside"? We have used aluminum for our frames since we started building robots. Yes It bends every now and then, infact we bent team 25's steel diamond plate in 2003.
The bends we get are nothing a hammer and fire can't fix.
Here is a picture of 2004 frame:
http://www.tigertrons.com/Pictures/2...n/100_0535.JPG
Edit:
We use 1/8'' wall thickness by 1.25'' angle aluminum.