You'll do that anyway, over and over. Especially if this is your first startup. In my experience (definitely including myself) most first time founders couldn't recognize a market if it hit them over the head in the early phase. The process of pitching your idea helps target what you're doing.
This doesn't change just because you've launched and everyone who's seen a little of the startup world knows to expect this.
Something I've noticed: I'm pretty good at telling other people where their product sucks, how they've completely missed the point, and how to do their job better. And yet I haven't yet been able to parlay that into a kickass product of my own.
If I reverse that finding and assume that I'm not particularly smart or special (usually a pretty good assumption), it follows that other people can do a pretty good job telling you where you're screwing up, how you've completely missed the point, and how you ought to do your job. And you're actually in a position to do something about it. And it's unlikely that they're in a position to do something about it. Hell, if they have more ego than self-interest (as most people do), you might even get this advice for free.
Definitely. It's painful thinking back on the first times I was pitching our stuff and how irrelevant the stuff I was saying was. Even now, 9 months in I struggle to make myself focus. Constantly pitching and having people rip what I'm saying apart is the only reason I'm not completely retarded about it.
This doesn't change just because you've launched and everyone who's seen a little of the startup world knows to expect this.