canuck, I would be careful doing that. Turning your gain and volume all the way up to get feedback can damage your speaker/s. You may not notice it right away because it's mostly a long-term problem. You amp is only equiped to handle so much, 10 is the max for a guitar thats not very loud. Guitars with humbuckers usually max out on an amp at 8-9. Pick-ups are only meant to pick up the sound of the strings. Feedback is producted from the pickups picking up the sound of the amp, and lopping infinitely. Since the amp is louder than the strings, the volume to the amp is magnified that much. So if your amp is at its max or over it's max, and then you add feedback; your pushing your amp way beyond its limit.

Some people when they need there amp up to it's max (because of live shows or whatnot). They will usually turn the volume of the guitar down half way when then go for the feedback. You'll still get the feedback without the damaging effects.