I don't ask them, but neither do I just book things.
For example, I'm going to Amsterdam in July with my boyfriend. I didn't know if my family had any holiday plans for them, so I didn't say "Don't book a holiday for such-and-such, I'm going to Amsterdam," but I said, "I'm looking at going to Amsterdam, is the first week of July OK or are you planning on going on holiday?"
The way you say things has a lot to do with how they'll react.