I guess it depends on how much they're able to change with patches. For the first three weeks or more, everyone's going to be on their "honeymoon" period with Smash (including 3DS buyers like myself), so no one will notice or care about any problem with the game short of a gamebreaking and easy to trigger glitch. As long as they get the core of the game right, like the physics, animations, music, and so on, it's not like the character balance has to be perfect on day 1. They also don't have to rigorously test for and eliminate obscure things in the vein of Melee's black hole.
So, yes, Brawl was delayed, but more recently Pokemon X and Y launched with a Lumiose City save glitch. I'm leaning toward them pushing the game out in time regardless. In the modern age, it's easier to apologize for a bug than a delay.