It varies so much & isn't consistent between stores in different locations. Waitrose is pretty much the most consistent store I think.
In Jigsaw there have been a few really awful ones who look at you as if to say you couldn't afford something that was a million seasons old & reduced 100000%, but there is one lovely one in my local one - well not all of the time unfortunately!
Waitrose people generally tend to be friendly, some in our local Tesco are nice, but the Tesco I went to in London was full of grumpy & unhelpful assistants.
Leia underwear shop in Cardiff - brilliant, so helpful & friendly in there.
Monsoon people tend to be friendly too in general.