I did cat5 everywhere in conduits, with the design philosophy that I wanted to have sufficient cable access to be able to change the cable in future. I'd probably use Cat6 now.
You can't have enough! Minimum of a connection in opposite corners of each room, always run 2 cables to each point (waste of time doing 1, you'll always want another). I also ran some to the garage....
You need ONE central hub with a patch panel 0 once done, you'll reap the rewards for years.
If you use 20 or preferably 25mm conduit, re-running cable will not be a problem.
I'm looking at replacing a few runs with fibre now to do some audio patching
Wireless aint too good through several layers of wall, and Gigabit ethernet for backups saves a lot of time over even 100Mb ethernet. I do use wireless, but just for convenience.
BW