I totally agree with Megatron. Though I don't think there will be a true starter per se, I do think both Shaq's and Perk's abilities are better maximized with the 1st and 2nd units respectively.
3 things:
1. The biggest problem with the first unit is scoring. During the playoffs especially, the starters go through some godawful droughts. As we've already seen in several games this year, Shaq's presence often ends these droughts or prevents them before they start.
2. In addition, don't underestimate the effect that Shaq has on first unit spacing...KG and Ray benefit the most from this! Shaq sets some of the best picks in the NBA, and there's arguably no player more dependent on good screen-setting than Ray Allen! It's a match made in heaven. For KG, Shaq occupies the attention of opposing defenses to such a degree that Kevin can now be aggressive in the paint without having to worry about double teams.
3. Shaq's abilities are maximized with Rondo on the floor (the other way around is somewhat true as well!) This is pretty straightforward. Rondo's passing ability allows Shaq to do what he still does best...FINISH. No longer does Shaq have to be relied upon to create his own offense. THAT was what doomed his experience in Cleveland!
Also, think about what the bench needs more of. I don't think it needs more scoring punch. What it needs is more rebounding and defense. It's the lack of these two things that has made the bench so unpredictable and occasionally ineffective. Perk (and Jermaine!) would provide these two things in spades.
In the playoffs, I think Shaq, Perk and Jermaine will split the minutes pretty evenly. Shaq will probably start, Perk will play big minutes with both units, and Jermaine will end games (especially against teams in which Baby's presence becomes a liability...i.e unable to guard lengthy bigs such as Gasol or Bynum).
Either way, barring injuries our bigs situation is pure gold. Need reliable offense? Insert Shaq. Need sure-fire defense? Insert Perk. Want a little bit of both along with great 4th quarter free throw shooting?...Jermaine's the man. This team's flexibility in bigs matchups will be ridiculous once everyone is healthy.