Let’s be honest here: your images and the work that you do should be compensated for if they’re going to be used commercially. While that doesn’t necessarily mean monetary payment, there should be some sort of fair barter. But with that in mind, you should always remember that specific term: fair barter.
When a company asks to use your images for free, you should carefully consider exactly what they’re doing. If you’re going to be part of some big campaign that they’re announcing, then they’re probably trying to take you for a ride. But if it doesn’t sound like such a big project, then they probably don’t care a lot about it and I’d suggest that you carefully consider that because you may not want to be part of a project that they put very little effort into. When you show that off in your portfolio or tearsheets, then potential new clients may not care about it very much.
Think about it this way: if a restaurant wants you to do a photography gig (shooting their food) for free, then you should barter accordingly. This is most likely a lot of work for you, and you should get paid for it. Let’s say that your rate is $350 for the first two hours and $50/hr after that, and let’s the actual shoot is three hours. Then you need to weed through your images and do the necessary work in post, and the job may end up being 12 hours of work overall. First off, you should consider whether 12 hours of work is worth $400 for you, which comes out to around $33/hr. But then if they want you to shoot for free then they need to find a way to compensate you for $400 worth of work.
You could try to get a running agreement with them because the company may really just not have the budget, but then in that case you two should really be held closely to that agreement. So, they should compensate you with $400 worth of free meals. That will mean that you’ve got some running store credit with them.
It also depends on how high end the company is: a local newspaper may try to take you for granted while the NYTimes is more likely to pay a photographer because they understand that good work should be compensated. Further, if you don’t deliver then they can make you redo it.
Either way, remember the barter system and if it sounds like you’re being used, then have more self-respect.