Coolant tempature

I have a '92 300SE (W140) with the 3.2L engine. During the winter the coolant temp gauge used to read approx. 80 degrees C. Now with the warmer weather the guage is running approx 90 degrees C.

1. Is this normal or should i change the thermastat?
2. I've seen other posts recommending drilling a hole in the thermastat, is that an appropriate fix as well?
3. Will i need to bleed the air from the coolant system if i change the termastat? If so, is there a special proceedure?

Thanks!