I was whistling into the mic and watching the meter. No deflection.
Should I see deflection when the mode switch is still in TUNE or only after I switch it to USB?
In TUNE, the mike isn't even connected, so you can whistle quite a lot and nothing will happen.
If you TUNE up properly, there is no reason to whistle into the microphone once switched to USB (or LSB). Just talk normally and adjust the MIC gain for proper ALC reading on the ALC scale of the meter. Whistling is very "liddish," and will identify you very quickly as someone who doesn't know what they're doing. Don't do that.