Charge for everything. Watching someone's cam is giving them special attention, which means you should be compensated accordingly. In my experience, giving away free attention tends to reinforce the idea that they are entitled to it. Which can turn into a give-a-mouse-a-cookie situation. It never really works out. And honestly... If you watched every dude's cam for free, you'd have no free time to focus on your own show or hustle. So only watching those who pay for the privilege has always worked out best for me.
One of the best rules of thumb that I follow is... If someone has a special request, it's usually best to charge for it. They want you to watch their cam? Charge for it. Do they want you to show your feet? Charge for it. Stand up? Dance? Play a special song? If it's not your idea, and not something you intended on doing in the first place, charge for it. If they want their request bad enough, they will pay for it. And if they aren't willing to pay, then they've proven that they only want freebies, and thus, aren't really worth wasting your time on. But this is based on my own experiences/opinion, of course.
Your time and attention are valuable - don't be afraid to charge for it!