Some great advice here. I'll add my 2 cents - hopefully it will help.
For a party of 7, some PS's are really advisable. It doesn't have to be part of a "plan every second of every day" thing, but setting up several meals as sign-posts for your trip would really be smart. Say, one every other day - more often if you are the type of people who want to do more sit-down than counter service.
It also depends on how fussy you are when it comes to time. If you need to eat between 5 and 6, chances of getting a PS for prime-time like that on short notice isn't good. Here, again, the large party is going to get you - if you were 2 people you'd have an easier time, with anything over that it's going to be much tougher.
I'd look at the great menus and reviews over at AllEarsNet and pick out some places you think would be fun, and sell them as experiences and not meals to your fellow travelers. In a few weeks when the park hours for your trip are available (as well as Extra Magic Hour schedules) it's a good idea to come up with a rough idea of which parks you might like on what day. As Nicole said above, you don't have to be at the restaurant just like you don't have to absoultely decide where you are going to be what day - but it does help to have a possible framework in mind before you go.
Also, this isn't policy, but I have found that once you have a PS it's a lot easier to get a PS. With a party of 7, I'm sure someone will have a cell phone (and if not, a Disney house phone will do - just ask a CM) - if you think you won't make it or want to adjust the time you are much more likely to be able to if you already have one. I've just found that works well in practice.
Making PS's ahead of time, with large parties and busy time periods (and especially a combination of both as you have here) can really be life-savers. Popular sit-downs (Brown Derby, Sci-Fi, Le Cellier, anything in Magic Kingdom) turn people away no matter how long you are willing to wait at times like those, and the rest of the places that you can actually get in to you will have a long wait. You may get lucky - but that's a chance you have to decide to take or not yourself.
I do understand the anti-commando mentality. I'm of both minds - I love to plan incessantly before I go and when I'm there I'm totally carefree. I don't think, "OMG! We have to get to Boma before 5:45 or the vacation is ruined!" I think, "Gee, it's 4PM. Hungry? Let's head to Boma - we've got that PS." I keep most of them this way, and it doesn't feel like a bother.
More options = better. Although I wouldn't make a habit out of it, in deference to other guests, PS's (with the exception of the small handful of meals that require actual monetary deposits) don't need to be cancelled - the system is designed to simply skip you and move to the next name. If you know in advance, however, it is always nice to call so other last-minute guests can make PS's in your place.
Good luck!
AEfx