I have a dilemma, yet another one.......LOL
Ultimately you want to look awesome for both the wedding and the reception.
If you have full highlights now you can likely do a half head in May if costs are an issue. And if you have your perm now, you may like how it grows out and not have to repeat it later, giving yourself great do's for both occassions.
Do it now, that way you look fab walking down the aisle (with no hair regrets) and then the flexibility to decide your needs closer to the reception. We all know us ladies get *****y when we have a bad hair day and you dont want that for your wedding!
Congrats on your big day and best wishes for a lifetime of love and happiness.