I would say so, yes. The aspect is going to have more to do with your source than your projector. You should be aware, however, that if you get a DLP or most LCDs (sans the Sony 10HT), you will be getting a 4:3 panel. So, if you watch 4:3 source than you will have some image either above or below (since you can zoom to not have both) of the screen. On a 1024X768 DLP projector you will be using 576 pixels to view 16:9 source unless you get something like a panamorph lens. Regardless the real question is what material you use the projector most often for. If it is 16:9 than you will be fine with a 16:9 screen. If it is 4:3 than you may want to stay with your 4:3 screen although a 16:9 screen will certainly work as long as you aware of its limitations.
If you get the Sony 10HT than you don't have to worry about either since it has a 16:9 panel. This is not an endorsement of the projector (as I decided against it), however, it is worth noting.
The Stewart is a great screen. I actually have a Da-Lite which I also like very much. Chances are with either LCD or DLP you will wind up with the same screen gain no matter the projector.
Lastly, you could spend much less money and mask off your screen effectively giving you the best of the 4:3 and 16:9 world. Just an option.
Best of luck.