Legal or not, I personally don't think any company is gonna give a hoot about a couple guys here and there using their wares for volunteering on a free project. If it were me, I'd be ecstatic that you'd use mine instead of something else, cause when you get hired by a company for being so awesome at modeling and you actually have to buy your software, wouldn't they rather you had trained yourself on theirs instead of a free alternative? Many software products have the 'free except for commercial use' policy in place, and I think it would do these other companies some good to latch on to that philosophy.