I'm surprised this isn't covered in your manuals. There is an important difference between standby and on. Assuming the mics you have work like many wireless systems I've seen over the years, the standby mode (which sometimes has other names like "mute" or "power.") turns on the wireless transmitter, but mutes the microphone's audio output. This can be an extremely important feature in circumstances where there is a lot of RFI (Radio Frequency Interference). Sometimes when the desired carrier signal is not present, a wireless receiver will pick up enough other interference to produce loud noise, which can be a pretty big problem in most applications. Many receivers have a squelch control to help set things up so that even when the transmitter is off no noise is output, but the likelihood of some stray signal getting in to your receiver is still much greater when the transmitter is off. Leaving the transmitter on, with a valid carrier constantly being transmitted, gives the receiver something to lock on to, which helps prevent it from picking up stray signals that can get amplified through your system.
If you are experiencing little or no interference and you wish to conserve battery power as much as possible, you can certainly turn the transmitters off during a performance. You are right that the "start up time" from off to on is generally about the same as from standby to on. But if your application/performance is critical and/or you do have some interference, and you don't need to conserve batteries, you may be well served by using the standby mode to mute the mic until you are sure the sound engineer has pulled the levels down out front.
Last update: 10:05 AM Saturday, February 25, 2006