How does grazing get in the way if you're in someone's house? It's not like you have to stop what you're doing to find a cafe. I understand not wanting your own DC to have biscuits and snacks, but you can teach them to decline if it bothers you (or brush teeth again if they accept!)
Marianne I don't think it's very nice for a guest to have to give a reason why they need to eat regularly. Maybe they don't want to share personal information about a health condition or they haven't announced their pregnancy yet (nausea/faintness is often worse in first 12weeks). Host could avoid this awkwardness by simply putting out crackers or plate of biscuits for all guests to share... then there's no need for anyone to feel singled out or embarrassed.
Kits I would never go in a host's fridge/cupboards uninvited, but then i've never been in situation where a host has restricted guests from helping themselves to food! Every friend I've stayed overnight with has shown me where tea, coffee, snacks, breakfast things and crockery are stored. I would think it very odd and inhospitable if they didn't do this. What if you wake up early and want to have a cup of tea and some toast? Is it fair to make people wait until a formal sit-down breakfast if they wake hungry at 6am? This sounds a dated way of hosting. IME people get up at different times, some want breakfast some don't, some want it early, others prefer late. People like some freedom and independence. When we have guests I put out a continental breakfast around 7:30am and guests help themselves as and when they wake up. Much less pressure. And less hassle for me. I'd find it quite irritating if guests expected to be waited on (and were dependent on me and DH preparing all their meals!)
Out of curiosity, why would you be uncomfortable with guests helping themselves from fridge or cupboards?? When hosting, the first thing I do is give guests a tour of kitchen so they know where to find things. There is nothing in my fridge or kitchen cupboards I wouldn't be happy to share with guests (if I had a special bottle of wine or chocolates I was saving, I'd simply store elsewhere). If there were things I was saving for cooking, I'd label them so no-one ate them by mistake.