Hi Peter, there are pluses and minuses for either a grandstand seat or just being on the crowd line.
Grandstand seating - my experience is based on using the FRIAT grandstand, which is located pretty much at the centre point of the crowd line. The advantages of a seat are, well, you've got a seat and a slightly elevated position. The elevated position is good for some things but IMHO, it's best for the Monday departures. The down side to the grandstand seating is that it can be pretty cramped if you're tall and a bit of a PITA if you have a lot of camera gear with you.
Crowd line - the thing to appreciate with the RIAT crowd line is that it's very LONG. There are a few places where you can't get to the very front (e.g. grandstands and other enclosures). If you want to get right to the front of the crowd, then you've pretty much got to get in early and stake your claim. Being at the front of the crowd is essential if you're looking for shots of stuff on the runway. For the flying displays, it's far less essential.
The type of shots you're looking for will dictate where along the crowd line you need to be. It also depends which direction they're using (although the wind is predominantly from the west, so runway 27 most often used).
In summary, there is no one "best place", a lot will depend on the types of shots you're looking for. Closer to the time, you should be able to download a show map from the RIAT site.
Grandstand shots (departures Monday):Grandstand shots (weekend):Crowd line (middle, not at front)Crowd line (near 27 threshold, at front)