I think I'm fairly laid back and don't have that many rules really but do try and make sure my DCs have a decent diet.
DS2 doesn't have much of an appetite and if he eats anything substantial after school won't eat his dinner, so I have developed a system of offering either very light snacks (fruits and/or yogurt) or actual real food when he comes in from school. If he has proper food, I know he won't eat his dinner but I cook less and at least I know he's had something good to eat IYSWIM
So he turns up tonight with a friend in tow. DS2 asks if they can have a snack and a drink. I said of course and knowing that this child is "allergic" to fruit weakened and offered some homemade cake. DS2 thought it was Christmas 
Friend turned his nose up and asked what else there was. DS2 has given him the last packet of Monster Munch (huge treat here as I only buy crisps approx. once a month). Friend is still hungry and asking for things that are apparently always in the cupboard at home but not here. More crisps, Twix, chocolate etc. I do have a packet of chocolate biscuits I bought to take to a friend's which I could give them but that would break all my rules re DS's diet. There is also toast, cheese, two types of homemade cake, yogurt and 4 varieties of fruit they could have.
I'm really starting to take a dislike to him (there are other reasons too) and not sure if my reaction to his food fads and requests is unreasonable or not. I'd hate to think my DS would behave like this in someone else's house but OTOH, I would do my best to make any other guest feel welcome and feed them what they like. I've stopped inviting him for tea as he won't eat anything that doesn't come out of box but I guess that's what he's used to and not his fault.
This boy is here a lot though (2-3 times a week) so not sure if he really counts as a guest. I would also relax my "rules" if it were a rare one off but it's not. If Ds eats badly because he's with his friend then he would rarely have a decent meal over the course of the week. (I know but try not to think about what he eats when he's at his friend's house!) They're 11yo. WWYD?