You should make it clear when you're making a ruling. Just stating rules without mentioning whether they're your own house rules or core rules means that you stating one of your house rules is syntactically indistinguishable from you misremembering a core rule. Furthermore, even if you are the DM, as someone with knowledge of the rules, I should point out things which you may have overlooked, since if you don't mention that you've considered a certain aspect, I have no idea if you've thought about it and decided to ignore it or just weren't aware of a certain thing. Nobody knows all the rules, after all. I know that as a DM in the past I've made rulings and then immediately overturned them when one of my players pointed out an exploit or other inconsistency generated by the rule I changed.