[Pogopuschel]

I think I usually hear お箸おつけしますか？ (ohashi otsukeshimasuka?)
To decline you can say 結構です。　(kekkou desu). I think いいえ、大丈夫です (iie, daijoubu desu) is okay as well.

Supermarkets usually have self-service microwaves but I never see them in convenience stores.