Think of examples of parents who say yes, whether you know them or not. Then look at the results and ask yourself, is that what you want for your child. (Totally different realm, but I think of Miley Cyrus and family.)
Kids do have a mind of their own and it's even harder when hormones are raging out of control but when a parent "caves" in to pressure or uncertainty about a decision, it's like blood in the water to a shark.
Isn't it ironic that when our kids are small we drill it in their heads to NEVER talk to a stranger, give them their name, get in their car or walk off with them. Give them another dozen years plus and we allow them to go to people's houses we know nothing about.
Yeah, I was a "bad" Mom who had no trouble saying no. I'm sure I'll be even a worse grandma when that time comes.