I doubt very much that you'll have an issue getting a mobile order time that you like on short notice at dinner time, but it's not impossible with the most popular places.
Woody's Lunchbox in particular can be hard to get right at lunch time, not sure about dinner. If you really want that particular location it couldn't hurt to secure a slot earlier in the day. Keep in mind that you can cancel or modify your order later if you change your mind - your payment won't be processed until you hit the "I'm here, prepare my food" button, so there's no real downside to getting a slot early, even if it turns out you didn't need it.
You can modify the time, or the items you order, or again, just cancel the whole thing.