The Raptor is a standard ON/OFF/RESERVE valve.
If I remember right (correct me if I am wrong) the stock vac petcock does not have a true "OFF" setting.... only ON and PRIME It relies on vacuum diaphram to "open" (when engine is running) or "close" the valve when the engine isn't running via the vacuum line to the carb. The PRIME setting is a straight "ON" condition and bypasses the vacuum diaphram.
Like it was stated before, if you leave the stock petcock on PRIME, you do not have a "reserve" capacity... when you run out of gas you are out. This would be like running a Raptor on "RESERVE" all the time. Hope this helps.
Yes, you should turn the gas ON or OFF on the Raptor. If you leave it ON by mistake, you should be okay IF your carb float needle is working properly, but I wouldn't leave it on all the time.... why risk it?
(Just an opinion from an "old school" rider)
I am not aware of any gas mileage affects either way between the Raptor or the stock petcock.