Depends on the game and what guidelines have been set out by the community. Generally, if it's in the config files and not in a settings menu, then it probably isn't allowed.

Generally Tampering with anything outside the games normal means is considered cheating. This is as per Speed Demo Archives rulings as well as many other places. Everything done to a game be it glitching or whatever you do must be done within the game its self and with no other tools or programs tampering with it, to be considered valid on a competitive level.

Any further game specific questions should always be answered in the games specific forums.