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F150 Head Gasket

I have owned a 1999 Ford F150 since it was new. It is a 4.6 V8 with 90k miles and (2 w/d). Once in a while, the engine would miss or slightly hesitate at highway speed. As a result, I had my mechanic change the spark plugs and plug wires. Four of the old plugs have a white/grey dry crusty deposit on the plug tips almost 1/16" thick. These were removed from both sides of the engine. The other four plugs are much cleaner. The mechanic claims the white/grey deposits are the results of antifreeze on the end of the plugs which he says is a telltale sign that the head gaskets are beginning to fail. He further stated a repair could cost up to $3,000. The engine is running fine now. Is my mechanic giving me a correct diagnosis? Thank you.

Which cylinders? Maybe 1-6-7-4… or 5-2-3-8? If it is either of those a head gasket is not immediately indicated but rather problem in the intake or crankcase venting to the intake.

Is the truck actually using any coolant?

The coolant is up to the top (cool fill line when engine is cold). There has been no coolant added to my knowldedge, other than 1 year ago when my mechanic changed upper and lower radiator hoses.