It depends on the game and how familiar everyone is with it.
Monopoly is already a complicated game and I think everyone is familiar with it, so a few house rules to speed things along never seems to hurt proceedings. Everyone I know seems to use the free parking cash rule (although in my house rules you need to have landed on it with the roll of a 6, so it's rarer), and there's a few extras I've encountered that just alter the game for fewer or more players, or who can't devote a weekend to it.
More unusual games will usually have us stick to the rules, because we won't have played enough to know where it's lacking. If one becomes familiar, then house rules get suggested unless the game is very, VERY solid built rules wise.
The biggest place for house rules are table top war games. With 40K, Horus Heresy and the like, where there are hundreds of rules and you have to build army lists through unit point costs, plenty of things get overlooked by the official rules, or are obviously miscosted after playing them. Agreeing something is actually 10 points cheaper, has an extra wound, or can only have 1 per 1000 points in a game, just flat out helps make the game better.
Same for customisation. If rules exist for a Librarian, bikes, power spears and how force weapons work compared to power weapons, but no official rules for a Librarian on Bike with a Force Spear exist, then you can build and point up with house rules to use such a unit in game pretty easily and not break it.