My partner and I are in the same situation. With heels, I am almost 2 inches taller. We were used to competed in American style (rhytm and smooth) and it never caused us much problem we were used to do pretty well in competition. We started competing in International last year. The only dance in Latin that is giving us trouble is Paso Doble but our coach is working things out to try not to make the height difference show too much. So far, our results at competitions have been pretty good. In Standard, it is another story. Because as the girl I have longer legs then my partner our waist level is not the same which result sometimes in a position and contact problem, as phil.samways mention, I really have to soften my knees. I also have to work hard with my posture and how to place my head, it might sounds weird, but for a couple without a height problem, if the posture and the head are good but not perfect, it will not show as much as a couple with a height difference.
When we started competing, we had the same concerns, but if your technique is good and you work on your style and even with a height difference you make it look good on the floor. It won't make much of a difference. So go for it and enjoy yourself!
Estrella