First, Gmail always uses push mail, and it can do it very efficiently battery-wise, because it uses a closed, custom, networking protocol and has a very smart mail server working with it.
Us peons (speaking of myself) have to use open documented protocols, e.g. IMAP, and the way to do push mail with IMAP is quite taxing on the battery (Steve Jobs thought so too).
In addition, Gmail's IMAP server limits the number of allowed simultaneous connections per account, and each folder in push mode needs one. Further, when the device loses connectivity, it takes time for the server to notice that, and "cross out" the now dead connections.
Because of all of the above, I recommend using no more than 1-3 folders in IMAP push (IMAP IDLE) mode, and keeping "scheduled mail checking" enabled, to cover your other accounts, and folders where push is not enabled.