The most important indicator of wear is oil consumption. If your bike does not consume any or just a little (let's say, below 0,5 liter per 1000 km), don't touch it.
Engine noise is a second, but when you are new to these bikes you'll find it mechanically very noisy, whcih does not help you.
Third: if your bike makes a noise like a German dive bomber in fifth gear, it's time to replace the fifth gears, for which you need to disassemble the whole engine, so then it is probably a good idea to also rebore piston and put in a new timing chain, as well as new seal on the valve stems.
In summary, no real reason to open it unless there's one of the three symptoms as described above or you are going on a big trip and want to make sure of your bike.
And remember: lot's of people have travelled on XT's and came home with a leaking, oil consuming and generally worn down beast. It does not often happen that they let you down!