I always find this an interesting debate as I have had this same conversation irl with several childminders and we all have differing opinions 
One for example had a parent insist on looking around her whole house - she was taken aback as parent was quite assertive and CM is not, so let her do so - this was for an after school placement where child was going to be downstairs whole time (cm has downsatirs toilet) - child never came to her in the end and she was stressing about being cased out for a burgarly for ages!
Another told me she would allow parents to do so as it "is their right" but only if asked - she was a new cm having been a parent using CM beforehand
My own view is that I always show parents around downstairs and advise them upstairs I have three bedrooms and a bathroom. My whole house is registered and the OFSTED inspector looks around every room unaccompainied by me at every inspection. If asked I happily show parents my spare room which is the primary room used for sleeping babies. It also doubles as my DHs music room and our guest room though so is not a nursery and contains other things such as musical instruments, sofa bed etc. I tell parents when more than one child is sleeping, mine and my daughters rooms are used.
I WOULD NOT however show these rooms as they are our only private space and I would politely emphasise this. The rooms are only used for sleeping children and i visually check for any risks every nap time.