If you're playing with the same guys all the time, there are few house rules, and no one's a rules lawyer, it's a non-issue.
Otherwise, it's helpful to write them up.
Rules are a social Contract
When you say, "I'm Running Pathfinder," you're effectively saying, "I'm running a game where the Pathfinder rules determine the way the game universe runs." It tells players what to expect and experienced players will be able to judge actions based upon those rules prior to your actually defining the individual rolls. Saying "I'm running the Warhammer world using GURPS" says a whole different set of paradigms; what kinds of characters by the setting, and how well they work by the rules.
If making changes to core elements, its best to write those changes up and give everyone a copy.
If you have new classes or class options, write them up for clarity. New skills, keep a list. New races, write them up just like any other race in the game. New ship design options, alterations in combat, advancement, character generation, or magic all should be written up and shared prior to play.
If making setting changes, include anything that the PC's should know about.
If playing Ravenloft, and Strahd doesn't exist, no big deal. If playing Ravenloft, but it's an area one can walk into/out from on the prime plane, that's a huge change, and players need to be aware of it.
If using a rules hybrid, note which rules are in use from which sources.
NB: This is extremely common in Traveller players. It's not uncommon in pre-3.0 D&D groups, too.
This solves a lot of arguments over what X book says in Y circumstance.
For example, in my last Traveller game: Tasks - Mega; CGen - Mongoose Playtest; Combat - Mongoose; Misjump - variant Mongoose; Psionics - Mongoose Psi Book; Trade - not used; equipment - Mongoose CR and S4; Ship Combat - mayday movement, Mongoose damage; Ship design - Mongoose Core plus house rules which see.
When consolidating Supplement Material, just list the source.
You don't need to list the whole thing, just where to find it.