i've done it several times. i just space them out far apart. drink lots of fluids and make sure i eat great meals for the next week or too (i try anyways, but you know, it's easy to skip...)
the last time was in october. in fact, that time they didn't even question whether i did or not. but, i guess that comes up when they want to know when your last period was.
i've only be refused once in the last 5 years, and that was because i had a low body temp. too low to draw, even though i offered to run around the bus for awhile. they did not like that. (i have low body temps normally)
if you want to do it, are in good health, and plan to take extra good care of yourself for afew days...do it!