First of all, go ahead, crop them if you want to. I don't care. You should do what works for you.
However, be aware that there are reasons for electing not to crop. They're pretty much all tied up with artistic process, so, have a little respect. If some chap has to quack like a duck every time he presses the shutter button, well, that works for him. That's part of his artistic process. You certainly don't have to do it, but don't give him a raft of shit about it. Unless he's a friend and it's like that, of course.
Among the reasons not to crop:
- Artificial limitations can be artistically stimulating. To argue that that your friend should crop is identical to arguing that he ought to paint or sculpt rather than photographing.
- Some people simply work better within a fixed aspect ratio, and find that the one offered in the viewfinder is the most convenient. Visualizing a final crop is a step they prefer to avoid, or perhaps are very bad at.
- Some people have philosophical ideas that inform their art, ideas that suggest that making a complete composition in the finder is the way they should make their art.
And on and on. There are tons of reasons to not crop. None of them are technical.
There seem to be, basically, two schools of thought.
The first agrees that there are reasons to not crop, not that cropping is evil or wrong (that is a common straw man erected by the second school, but essentially nobody actually thinks that). Some of these first-school people try very hard not to crop, others crop like fun.
The second school of thought thinks that not cropping is just a ridiculous and incomprehensible stance. These people are blockheads. Given the lack of respect for other people's artistic process these people exhibit, one cannot help but wonder about their own artistic process.
So crop if you like. Or think about it, and see if perhaps not cropping might help you in some way. It doesn't have to be a clear and definable way, maybe it's just a feeling. If you're curious, try it out.
Me, I crop all the time. Sometimes I crop heavily just to get some low-fi grit into the picture.