Yes they do tighten up on there own! And yes you can do damage to the valve seats and guides by having them too tight. If the valve wont fully seat then you will get blow bye causing loss of power if the exaust valves are too tight and backfiring through the TB if the intake valves are too tight as well as loss of power.
Just take your plastics off and adjust them! It's only about a half hr. process! Maybe an hour for your first time.
+1 i aint mechanicly inclined and it took me about 45 minutes i dont really remember but it was fast. Adjusted them using MPLeaver's thread on valve adjustment great guy very nice thread and alot of details you might want to ask for some help from a friend to hold the valve to the proper specs while you tighten it to the proper place you want it. I dont remember wich side tightens over time and witch loosens but the side that tightens over time i set a little looser and the side that loosens i set a little tighter. My rappy runs awesome no complaints i double and tripple checked everything. I did this when i was doing my oil change also. One thing is those gas tank bolts one sheared right off my bad but got lucky and had a bolt i could use on it. So dont over tighten. Your heart will skip a beat if you do LOL.
For my job i use some long nosed vise-grips with a small square head screw put it on top of the valve checked my measurments with the fealer gauge thitened while buddy held it in place with the screw and vise grip and voilà checked all the specs again turn the motor over 20 times by hand with the ratchet aligned the marks on the case and re-checked with fealer gauge PERFECT closed everything and couldent be happier i did it myself.
Just follow MPleaver's thread you will be fine. just tripple check all your specs.