I'd say that it depends entirely on the context of the bug and how it is used. Say there was a bug in a shooter that gave you infinite ammo, if you used said cheat in ranked games and just spammed a weapon that is normally restricted by the amount of ammo then I would say it isn't fair to use it. But if you play in a private, unranked game then by all means it is ok as you are just playing for fun. But it also depends on whether it is a well known, accessible to everyone type of bug. There is a bug/glitch/secret code in Burnout Revenge that gives you a boost start and can give people a ridiculous lead in a race, it's available in all races but can be difficult to pull off and probably isn't widely known (without scanning the interwebs for it and practising). So I always hate it when people used it online.
But if it is just something that can make private or unranked games a bit more fun or single player games fun (like infinite ammo cheats in GTA games, or invincibility in half life) then I wouldn't say it is cheating.
But if it is just something that can make private or unranked games a bit more fun or single player games fun (like infinite ammo cheats in GTA games, or invincibility in half life) then I wouldn't say it is cheating.