Do not worry about the RAM usage. RAM is there to be used. There is no point in the system keeping lots of RAM free, this makes the system perform worse. The idea is that as apps are used (either started by the user or automatically started when an app is woken etc) they are of course using RAM, the system keeps them in RAM so they can quickly be accessed next time they are required.
If the system closed all apps etc to free RAM you basically end up with more free RAM, but for what purpose. No use it sitting there empty - may as well go and buy a phone with 128MB rather than 1024MB if you dont want to use much RAM
If you start an app that needs a lot of RAM (more than is currently free) the system just removes older apps from RAM to free up the RAM required.
It is not worth comparing your phone's normal RAM usage to another phone, each person has different apps and uses the phone in different ways. It makes no sense at all to compare two phones where RAM is concerned.
Obviously there is the possibility that an app has a memory leak and starts to eat all the RAM up, but you would then most lilkely notice something was up with the phone, eg it slows down, things stop working etc. At that point you would reboot and note the RAM, then use the phone and monitor RAM every so often, at some point you may noitce RAM use going up and up to 100% - hopefully you are using a monitoring too to tell whats eating all the RAM and you then delete the app.
If your phone is running fine and you have some free RAM, even if just a smallish amount, then most likely things are running just as they should.
I am a UNIX sys admin and we have AIX (IBM's UNIX) servers with huge amounts of RAM, eg one Im looking at now has 130GB of RAM and its 98.7% used. Thats a good thing as we are making good use of the expensive RAM, if it was 50% used we'd be wasting 65GB of RAM which costs money.