Anytime there are means to present potential members join options that do not properly credit the referring affiliate, it is shaving. I agree with Dave that it should be included in the programs terms and conditions.
It isn't the most difficult thing to check for, so I would suggest any affiliate check out the program they are promotion for things like this or traffic leaks. Know the number of popups your surfers will see, and what those popups are. Are they being presenting with another option to join that removes affiliate credit?
This is one of the benefits to a program that cascades it billing is the surfer is automatically presented with the next option for joining. If all works correctly, the affiliates will receive proper credit regardless of the billing method the new member uses. I cannot say in all cases, since there are few custom solutions used that I know very little about.
Never make the mistake as an affiliate to assume all programs operate the same or even in similar fashions. Get to know the principles involved in the program. Understand the their term and condition, and by all means ask questions
