There is no "100%" answer.
First, players should be in AAA/BL when they're on the 40(4th pro season). There's no reason a legit prospect should be lower than that at that point.
Second, some prefer 1 level at a time while others don't really care about that at all.
Finally, if a recently drafted/signed player is BL-ready, there's no real reason to hold him back if he can help. However, you lose salary control much sooner. As it stands, you get 3-4 minimum salaries, 2 arbs and 5 LT years at a minimum. Some people prefer to hold players back to get all "peak" years.