I think it depends on where you want to go. If you are mostly interested in AOA programs, then forget about the USMLE. If you are desparate to get into a very competitive tier one program that doesn't have a history of accepting DO's then you'll probably need top USMLE scores on Step 1 and 2 and a great audition interview to get the spot. If you'd be happy with a great residency at a program that has a history of taking DO's, I think it's more important to get a rotation at that facility and perform fantastically than to get USMLE scores. The resident's I've talked to about this say that they notice those who took the USMLE (and passed) got more residency interviews, but that it didn't really matter when it came down to match day. You could even call programs now and find out what their requirements are - some will probably tell you that they don't interview unless you take the exam.
My only other thought on this is if you decide to take the USMLE, work hard to get great scores. Low scores on the USMLE will only put you in a greater predicament - do you release them to programs or not? Many of my friends are facing this now - high comlex scores with not so hot USMLE scores...
I'm like Flea girl though - these thoughts could all change come match day. BTW - I have great COMLEX scores, no USMLE and am getting interviews at competitive programs in SoCali not known to take DO's - so that's at least a positive sign. My reasoning was that my USMLE scores wouldn't look as good as COMLEX, since I spent all my time preparing for the COMLEX, not the USMLE. Hope all this rambling helps!