People are going to tell you different amounts.
According to Oldfishlady, who was a moderator that left the site, her recomendations for a 5G unfiltered were to do one 50% once a week and a 90% once a month - both with gravel vaccuming. With a siphon, you don't have to remove the gravel since the siphon will suck up any debris in the gravel.
If you cycle, then you can cut back to 25-50% once a week AFTER the cycle completes. It's really not as scary as it sounds. Basically just do two 50% water changes a week till the cycle completes OR you can buy the master test it and test for ammonia and nitrIte. When they get to .25 do a water change. When you get 0 for ammonia and nitrite but are getting nitrAtes, you are done.
As for cleaning decor - for the unfiltered tanks, I rinse the decor off in every water change. In filtered tanks, every now and then. the Baccteria you want to grow will attach to the decor, tan walls and substrate but most will be in the filter cartridge or sponge.