Sadly, anyone can gazunder, I'm worried about it in my case too.
Did you see the post upthread where someone said their vendor had covered up cracks? It's just important to make sure all is well. I realise that if you're packing and everything is nuts, it's not fun to let them in again, but it's good sense to make sure the property you are buying has stayed the same.
We often buy after a couple of ten minute viewings. I think you're taking a bigger risk by refusing them.
My sister had to measure up to be sure about a sofa fitting in. Otherwise, in a chain, you have to organise leaving it behind etc.
I try to be decent about stuff. The fast setting on my washing machine stopped working in my last place, between exchange and completion. (The long settimgs worked fine). I told them that and offered £200. I gave them a cheque direct to avoid disrupting legal paperwork.
I know some people might think it's dumb but I think, do unto others etc.
The people I bought from before had to sell in tough circumstances. But when I walked in after completion, the place was spotless and they left a "new home" card, a mini orchid, and useful local phone numbers. I plan to do the same for my buyer. I still think he's really an investor, but it seems like a nice way to pass on good vibes.