I have worked in gaming, and so have many of my friends (some of which still work at a few of the big names in the industry), and i can honestly say I know nobody (who’s a professional) who models game models with subdivision surfaces. Even the modern games that are very high poly count, subdivision surfaces just aren’t efficient. It would take far longer to model something in subD’s just to repurpose the mesh later for low-poly counts. Just model in straight polygons, preferrably in tri’s. That way you have full and complete control over every poly’s placement. Decimator lowers polygon count, sure, but not in an accurate or geometry-perfect way. If you want to make game models, do it the right way.
I mean, if you have some ridiculously high poly count limit, and the game engine is powerful, I suppose you wouldn’t have to care. =) Or if you don’t mind spending a lot of time re-creating sections that decimator didn’t treat too well, that’d be fine, too. If you’re doing something <3-5k polys, i’d still make every polygon by hand. That way you know exactly where your budget is.