As others have said, I expect that most of the raffles are legit and a few aren't. I agree that most of the raffle prizes aren't enough of a big deal to make it worth cheating, and hopefully the majority of models are honest anyway. Occasionally there will be a raffle for an expensive prize or a "date" with a model. I can see where there'd be a motivation to give those prizes to regulars/people the model's fairly comfortable with, so maybe a few of those are rigged (I've never been in one of those).
The bottom line for me is, I'm not that concerned about getting stuff, so I'm only going to enter a raffle from a model I would have tipped anyway. I view the ticket cost as just a tip and a show of support for the model's event. If I happen to win, that's just an unexpected bonus.
I guess my advice would be this: For small prizes, probably safe to enter since it's not worth the bother or the risk of a ruined reputation to fix the raffle.
For a large item prize - the model's got to make a profit on the deal, so the odds don't favor you. Only enter if you want to support the model anyway. If it's someone you don't trust or you're really just hoping to win the item, you're probably better off just buying the thing for yourself. For a date type prize, it seems to me that it's only worth entering if you trust the model. Do you really want to meet someone who would cheat you anyway?