The spacing on approach varies; runway configurations, wake spacing, noise abatement, types of approaches and missed approach paths etc.
Keeping a concept of 5 miles may work at some airports but at many others this is a 'fat gap'. The controllers may be banking on the current gap reducing, I know I certainly did when I worked on Approach. Helping yourself to a slow down to help us out may in fact make it more difficult for us. ie force us to slow the one behind you when everything was fine until you reduced your speed and then asking you to get off the runway quickly.
However, you are able to fly your own speed once cleared for the approach unless a speed restriction has been applied; so technically you're doing nothing wrong.
Rather than slowing without reference to ATC, why don't you ask if they would like you to start slowing too; or perhaps ask what gap they are looking for, it might be 3 miles or 2.5 miles?
Always better if you can to make a phone call to the relevant control facility if it's a constant location where you are having issues.