Yoda wrote:but it really depends on the type of league that you are in.
I'd agree with that. In my main league I'm 99.99% sure that no managers would cheat, and I'm also pretty confident in each manager's intelligence, baseball knowledge and fantasy "skill" that I don't think any trade will be vetoed at any time in the future.
In public leagues Noobie McNooberson trades Arod, A Soriano, Carlos Lee and Hunter Pence for Josh Willingham, Joe Crede and Jeremy Accardo and I'm all over the veto whether or not I have any proof or reason to think the two are cheating. In that situation it's enough that the trade is so lopsided that it completely destroys the league balance to allow.