I was going to stay out of this because I don't really agree with Late Fees and Cancellation Fees, but we HAVE put them in our Supps (for Atlanta Region events) in the past.
When we first implemented Late Fees (and the language is "MAY be assessed") it was because some folks (the same ones each time) were abusing the system and ALWAYS registering at the track. Not only is this a drain on the Registration and T&S staff, but it also can affect group make-up as well. Quite simply, you registering early gives us more time to plan adequately. I will also note that Atlanta Region does not deposit checks for our Road Atlanta events until after the event is over, so there is no penalty for registering early.
By adding the "may be assessed" language, we give our registrars the authority to "penalize" the jackasses that perpetually show up at the late window demanding that they get special treatment. To date, I'm not sure they've ever done so but at least now they COULD if they feel it's warranted.
For the Barber event we added a $150 Late Cancellation fee for the 2008 event. It rained on Saturday at the 2007 event and we ended up refunding over $10,000 in entry fees from people that were already at the track but decided not to go out in the rain. The feeling was folks might be more reluctant to withdraw if they had skin in the game. For 2009 I reduced that to $50, and even though it rained on Saturday (again) we only had about 30 drivers (of 180) choose not to go out. Hopefully I can convince the PTB to drop it altogether in 2010, but again the idea is to keep folks from deciding to not show up because they suddenly find a need to rearrange their sock drawer.
Again, we don't assess either fee in the case of truly unexpected circumstances - it's really just there to give some leverage against those jackasses that show up with bad attitudes.