As Frosty pointed out, a lot of older games simply don't work with higher resolutions without workarounds and even with tweaks, some older games function very poorly (enemies "popping in"/disappearing on-screen). When I play older games, I tend to keep the aspect ratio the same as my current monitor and just use whatever is the highest resolution for the ratio available. That tends to work pretty well for the most-part.
Also, for the wide-screen tweaks, playing Brood Wars and Warcraft 2 at a high resolution is basically cheating. I was able to view almost the entire map while messing around with the custom resolutions and the units were so small, I could barely operate selecting units.