I do exactly the same even in a retailer who has Apple Pay decals in the Windows/at the Point of Sale.
I will say out of the places I've used my Watch my Tesco store is by far the quickest and easiest in terms of paying/processing the transaction.
Yes, it's good that the terminal is always live looking for a contactless card/device. Do you still have to choose finish and pay?
A metro store I sometimes use was a trial store for contactless and at that time the implementation was not as good as you had to select contactless from the payment options.