He should turn heel before WM. It wouldn't work the other way.
Him turning heel is a huge deal do you really think they'd do it before WrestleMania? Besides if it ever happens they probably want it to be like Austin's heel turn, cept way more successful.
__________________ My Megapost (Updated 02-27-2013)
People complain about newer talent not getting over, but what they mean is that their favourite isn't getting over, everyone else can go to hell. I'm for as many people getting over as possible, it would improve the show and the more over people there are, the more avenues there are to push new talent, yes, your favourites are more likely to get pushed if there are more over people to feud with.
In all seriousness, the WWE title is already on the line. The match doesn't need anything else added to it. Besides, I'm almost positive that WM30 will be the conclusion of Rock/Cena story and I'm sure there will be an added gimmick next year to sell the end of the feud.
The only way it could get my interest is with Cena turning heel. Cena is already getting booed by 60% of the crowd, he would be over huge as a heel and taking the title off the Rock at WM would cement him as a mega heel.