I guess I would distinguish between four different kinds of “house-ruling”

Resolving a rule dispute

Of course, even in friendly games, you get to a point where you and your opponent are like “umm, not sure how to interpret this”. In this case, picking what seems sensible (or rolling off if it’s impossible to figure out) is the best option mid-game. I prefer to read the rules, FAQ, online discussions AFTER the game to try and figure it out. That might lead to a longer-term agreement that isn’t the same as what was agreed that game, and this might be a “house rule” from then on. Though I would likely ditch the house rule if later errata came out that clarified this issue.

Building your own ruleset

Obviously, if you decide you want to really start to make significant changes, not just to fix a few things you’ve struggled with but really re-tool it, that is a different beast. That really starts to come down to game design, balance and various other things. I think this is fine too, but it’s not really house-rules so much as a house-rule-set, that might once have been based on 40k or Infinity but has long since morphed into something else. It might be great fun or fit your groups’ needs, and that is cool. If there aren’t legal issues it might be nice to distribute so others can enjoy it. This isn’t going to set you up well for tournaments, but who cares if that isn’t what you want.  I would make sure anyone new joining the club understood though that what you play isn’t really the “normal” version of whatever game you started with.

Scenario Custom Rules

I think this is the area I think is most fitting for house rules if you are trying to build up a scenario with a particular narrative, this is a perfect place to do it. It’s also not having a knock-on effect on learning the core rules of the game, or balance forever more at your club. It’s just about making that particular scenario feel special.

Rule of Cool

You want to do something cool, and the rules don’t quite have a fit for it, so you’ll make something up on the fly. It’s a funny one this for me because I love RPG’s where they often have a pretty good generic mechanic that is like “if you want to do X, make some sort of skill check to see if you can do it or not” and it can work for almost anything. So the rule of cool is often easy to apply and fits within the overall intention (telling a great co-operative story). I find it trickier for wargaming where, whilst I certainly want it to be cool, and tell a story, I also want to feel like it’s a mechanically interesting game, and abuse of the rule of cool can really lead to a situation where neither player really feels like they have a handle on how the game actually plays.

I would also say, just in general, I do want to trust the game designers at least for a good few games, before I start meddling. They’ve probably put tons of effort into play-testing and tweaking, and it might not be until you’ve played a game a good few times before you understand why they’ve made certain decisions. I mean, one of the great things about this hobby is, it’s yours, when you and your mates are together you can do whatever you want and whatever seems fun, which is great. I just caution against getting straight in and hacking away at a game too early, or you might miss out on the gem of what that gameplay is all about.