When using a heatsink that's on a vertical T220 or T247 and that rests on the board below it, should you clear the copper under the heatsink? I'm thinking this type of sink. I mean the transistor SHOULD be isolated from the sink (if you use an isolating washer), so it shouldn't be a problem. But is it good practice to do it anyway?
It's isolated if you ensure that the clip is isolated too. With screw on heatsinks, you use a nylon bushing (not unlike the cheap LED holders) to isolate the screw. With this clip, I don't know if it is, but you could use a silicone TO-220 isolating washer to go in between the spring holder and the tab.
The heatsink itself can be grounded as long as you ensure complete isolation (check with an Ohmmeter if in doubt.