After 18 months of not flying, I'd suggest that you don't fly without an instructor - whether it's now, or when you get to America - until the instructor is happy with your flying. 18 months is a long time - I tend to go rusty if I haven't flown for 1 month!
I suspect that's probably why the school said it wouldn't make much difference either way - I can't remember what the exact requirements are if you go 24 months without having logged the required 12 hours in the last 12 months (one hour of which has to be with an instructor, by the way) - but I'd have thought that, either way, you'll need a few hours dual to get you back up to a safe standard again.
Bearing that in mind, I'd guess the main reason for doing some flying right now is that it's fun!
I'm half way though my ATPL exams, and I don't think that being current will make the exams any easier.
This is all IMHO, of course.
FFF
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