GMAT scores are good for 5 years so there's no reason to wait, plus you want to give yourself time to retake it if you don't get the score you want. If I was in your situation I would start my 2-3 months of serious prep now and plan on taking it in June or July. That way you could retake it in mid/late-August if necessary and still have over a month to devote to your R1 applications. Essays will be nerve-wracking enough by themselves--you don't want to be juggling GMAT prep with essay writing.
Then again an exact answer all depends on what score you're shooting for, how close you currently are to it, how much time per day/week you'll be able to study, when you'd be available to take the GMAT, what the R1 deadlines are for your particular schools, etc.