Diesel injectors are a crapshoot at the best of time. if you want to see if they are a problem, the best test you can perform is a leak back test. All it requires is a very cheap test kit to do this.
If the levels are about the same, you are good to go, you only really need be concerned if the levels are very different on on or two injectors.
JeanLuke's advice is good, if it ain't broke don't touch it.
As for injector cleaner, I'm a fan, they do help keep the system healthy. I use the product these guys sell, it seems to work -
http://www.pmlubricants.com.au well.
I don't mind this one either -
http://www.chemtech.net.au/cr_fueladditives.html
There are many people that have had good success with eh Fuchs injector cleaner too, it is more work to use though, but it will have more immediate results.
Jason