I suppose people have different opinions on this matter. What's ok with one applicant/program may not be ok with another. Anecdotally, I have heard a PC express displeasure at an applicant cancelling, so I don't think it's fair to assume that programs are universally ok with applicants stacking invites.
While not rad-onc, I found these comments from a PD interesting when doing a google search on the topic:
http://urologymatch.yuku.com/forum/viewtopic/id/364
Different programs are clearly going to be bothered by different things when it comes to cancelling interviews. Not intentionally double booking avoids the problem of having to swap or cancel.
Also, imagine showing up to your interview and when they ask you about your other interviews being able to say "well, I got a late offer to interview at <top program>, but it was the same date as this one and I really wanted to talk to you guys."