Having experience in the US is very important, the reason being it gives the program a chance to see you in the field as opposed to hear things about you from a letter or recommender by phone. It also reflects as to how serious you are about a program if you are willing to come and do a rotation there.
True it is not a must, but it makes alot of a difference from the programs perspective if they saw your face and got a feel for you, if your scores are on the lower end - you should definately aim for US experience, if youre going for a competitive residency you should also gain US experience. In short - if you have the option - do it, it will help you 100%
I always recommend reading
www.imgresidencytips.comron