I tip -
The Walmart I use is in DE, there is no signage that I've seen physically at the pick up location. Haven't noticed anything online while submitting order, although admittedly I never thought to look.
I tip in cash, a few bucks for regular orders. $10 at the holidays. Never had it declined, but it is always gratefully accepted and sometimes with a bit of surprise, which makes me think that (maybe) not to many folks tip them. Considering they are schlepping all that stuff out to my car, sometimes in inclement weather, and loading it, tipping seemed appropriate.
*tipping seemed appropriate to me, no please this isn't the place to debate the merits of tipping in the US. It's just an accounting of my experience with tipping at Walmart when picking up groceries*