Eh, it all depends really. There are so many rumors, rumors about rumors and general speculation regarding the main event that it's hard to really know what might be the right thing to do until WrestleMania happens.
For instance, IF Brock Lesnar is staying in WWE and does retain against Reigns, then Rollins cashing in on Lesnar, screwing him out of the title and feuding with him for a while could be very entertaining. Aside from generating a ton of buzz, it'd put the WWE World Heavyweight Championship on a very over heel that's on television each week.
IF Lesnar is leaving and Reigns wins the title, it depends on some things. I don't think Reigns is going to be the ultimate, over babyface that Vince hopes, though that might not stop him from getting a run with the title, it might keep him from being Cena's eventual successor. If Lesnar loses to Reigns, I see them giving Reigns his moment in the sun at WrestleMania. Lesnar will be guaranteed a rematch and unless he signs a new deal, his last contractual appearance is the post WrestleMania Raw. If he intends to leave, I see him getting his rematch on that episode of Raw to help pop a rating as it'll be the first time he's wrestled for free on TV in over a decade. Reigns retains after another hard fought match, Rollins cashes in, screws Reigns out of the title with Reigns chasing the strap while possibly building up his credentials along the way before regaining the title at SummerSlam.
The next MITB ppv is to be held on June 14, so there's not a whole lot of time left to cash in and the times in which cashing in generates the most buzz are WrestleMania and the post WrestleMania Raw. If he doesn't cash in, whether successfully or not, at either event, he'll only have another 10 weeks before the MITB contract is null & void.