For bobl you don't use plumbers grease on o-rings. Bad for the rubber. Silicone grease is called for, and for a one-time deal like a tub spout, soap won't hurt.
Plumbers grease is meant for metal parts, like stem threads,cloth-impregnated packing, bonnet threads, etc. It has been widely used as a general purpose lubricant, but in the "o-ring world" it is not appropriate.