I'm replacing my MAP sensor and on the old sensor, around the base of the nipple and on the orange seal, it looks like it's had grease on it. Is this necessary, like for heat purposes or something? Or is it just 200,000 miles worth of build up?

 

most likely just oil residue buildup.

 

IMO a applying little grease on the seal will alow it to seat properly

 

It may seal it a bit better. Just make sure you don't get any in the opening on the sensor, or hanging down into the intake manifold. Make sure its grease that won't deteriorate the rubber seal. Properly installed, a healthy rubber seal is usually adequate.