My personal style is to start a player in Rookie (only exception would be an IFA signed early in the year before RL starts in which case he starts in LoA) and advance them one level each year. So they spend one year in RL, one year in LoA, etc. Every prospect will also start AAA for a second season (usually on the inactive list for position players to save AB's) and move them up after 25 games to gain an extra pre-arb year. The only exception to this is with a lower-level prospect who needs to fill in for the first 25 games while I am waiting to call somebody else up. Or if an injury happens or whatever. But even in that case, you can usually find a decent stop-gap off the waiver wire or free agent heap.
My logic for this strategy is this:
By bringing a player up early, you are basically trading in a year at the back end of your team control (when they are 30 or 31) for their season now, when they are 20 or 21. Generally speaking, a player is going to be better when they are 30 or 31 than when they are 20 or 21. Sure, bringing him up now might help you this year, but you also might wish you had an extra year of that player when he hits free agency.
It's personal preference, but for a newbie, I would definitely recommend you be somewhat conservative with promoting prospects to ML.