I was always very pleased when my mum did jobs when visiting. I knew that she was doing it to help me. She came to stay for a few days after the birth of DS1, and told me then that she would try not to overstep the mark - apparently, when her own mum came to 'help' her in the same situation, she started by cleaning and rearranging a display cabinet!
I regularly do things at DS/DiL's house - washing-up, weeding, cleaning windows), but I checked in the early days that they didn't mind (was babysitting at time), and explained that i find it hard to sit still for long. I also told DiL that she shouldn't be afraid to let me know if i went too far. No complaints so far ☺
That said, apart from a bit of washing-up, I don't usually do anything when there as a visitor, as opposed to childminding. (Mind you, it's quite rare that i am a visitor!)