In general, yes. That said, I look at a new rules system pretty carefully to see if it's something I can live with. I try to use the tools that best fit the job, to reduce my workload. Time is better spent on worldbuilding and npc creation than writing new rules. If I run fantasy, it'll probably be D&D. If I run a modern game, it'll probably be GURPS. If I do a superhero one, it'll be Mutants and Masterminds. A Wild West game, I could do in GURPS, Sidewinder: Recoiled, or Deadlands d20, depending on the flavor I wanted to impart.
ANY flavor could be done with ANY of those systems but for some, I find I like the different rules sets.
If I do houserules at all, they are generally 'flavor'-based ones such as changing a racial ability to better match my idea for that race, or dropping a handful of spells that either I don't like (I've only met three spells I don't like, though I have not even glanced at the ones in most supplements) or do not fit the setting.