I cant remember, but I thought Apple and Yahoo had something going where as yahoo was treating all Apple accounts as IMAP and were actually pushing e-mails to the phones when a e-mail was received at yahoo, anyone ever remember hearing anything about that when Steve Jobs had brought the Yahoo CEO on stage during the iPhone intro? I very well could be wrong... But either way, Unless you MUST have that e-mail really fast, I personally wouldn't go below 1 hour intervals, Would think that would kinda hit your battery pretty hard considering it would check all your e-mail accounts ( If you have more than one in your phone.. I have 3.. ) every 15 min in a 24 hr period would be... what somethin like 96 times in a day , plus normal usage..
Me personally, I check my mail on my breaks and what not, and soon as you go into the mail it updates..