There was a girl that worked here at the NIH that I knew that graduated from Stanford in 2.5 years, then did an internship here for a year, and applied then. She left in may to do into the UCSF MD/PhD/MSTP program. Personally I wouldnt apply right away, take some time to get some real world experiences and take a break from the studying. Personally it felt really good to get away from the studying and refocus, and not get burned out. After 2 majors and 4.5 years of undergrad I needed some time. That would be my advice, but do what you you think will make you happy....
Jim