It will depend on QS vs TS and also how many people you have in your party. There shouldn't be an issue at all with QS - with the exception of possibly a line. However, if you have a party of 6 or 8, you may have a long wait for sit down.
My DH and I usually take a late lunch (1PM) because we're on CST and that's Noon for us. Since there are only 2 of us, we usually have no issue with TS for lunch.
It will also depend on where you go. For example, I would never think of going to the 50's Prime Time Cafe without a reservation, even for lunch. They are usually booked solid. The only exception is if you sit at the bar - but then you don't get the experience.
My recommendation - if there's somewhere you'd really like to try, make the reservation. If not, then you can always play it by ear for CS. Good luck!