I believe that the "early admission" to BB is only to grab sunspots. Not too sure though.
And very very few people go to the ERTs. I think most people don't even know they exist. They don't go in Action Zone, anyway. I'd say its no more than 200 people on an average Thursday morning - including the employees.
To actually answer dude's question, any time during the week is good. Just look out for special events (concerts, bring-a-friend-free days, Xtended play) that draw lots of crowds. Middle of the week is usually pretty good. You usually won't run into much trouble on Sunday, either. Don't go on a Saturday if you can help it.
One more piece of advice, start out in the Rivertown/HB area, and go counter-clockwise. AZ gets crowded really quickly, the crowds there start thinning out around 2:00 generally.