You noticed the main reason for not forcing a resolution: You never know FOR SURE what DVD will have which, and some DVDs have 2 on a SIDE even, so how do you determine?
NOW...all that said...the technical jargon is VALID, but often overanalyzed...remember that the DVD format does not EVER exceed its base resolutions, or change for single scenes, its fixed per title(Menus dont count though!). For basic PC user theory, its actually ALMOST ALWAYS slightly between 800x600 and 1024x768 on most systems(About 95% of all discs). If you increase your screen resolution to 1280x1024 or 1600x1200, the image will get more "DEFINED" as you will be able to see more of each pixel(Since they will be enlarged/stretched so to speak), but the sharpness may/will actually DECREASE, because the number of pixels is a constant so you are basically stretching too few pixels to fill extra screen space/pixel area you added by going to a higher resolution. Like looking at a picture painted on a balloon before and after you blow it up...it gets bigger and you can see tiny details closer, but it loses even MORE tightness on lines, image loses clarity, colors fade...etc etc.
I used to generally recommend 1024x768 to people. A good general resolution for most DVD viewing, and at that resolution, you almost always "See it all" resolution and pixelwise(On most titles)...WITHOUT too much stretch effect you sorta get at the higher resolutions on lower resolution titles. In fact the true resolution of most NTSC discs I reviewed was closer to 800x600, but just slightly over that. NOW, could I possibly see the difference? having watched over 200 discs while doing reviews(Including THE MATRIX over 100 times, gawd I was getting bleery on that film!). HONESTLY, Id have to say NO on most fullscreen titles. If you stretched to fullscreen view, you usually couldnt tell the difference in clarity or sharpness, or color precision. the effective difference IS in fact real and there, but MY human eyes usually had alot of trouble picking it out when I was hunting it (The difference between a DVDs clarity at 800x600 vs 1024x768 when checked in color matching and line sharpness is what I was checking). WHat Im saying is, and this was also brought up by an engineer at Cyberlink who called me about a review statement...the VISUAL SIZING vs PIXEL VOLUME (How big a resolution your screen is vs what the DVD truely has to fill it with) doesnt equate to CLARITY by itself. At close resolutions, size being equalized(2 windows same size), you wont see any clarity difference in and of itself on single resolution steps. the human eye CANNOT see a single pixel that is 5% different in color from all those around it (Thats a test they use). NO HUMAN EYE can tell where on the screen that one pixel is. At ANY resolution.
NOW...if you want to analyze PER DISC (Yes there are a few titles out there that buck norms) youll find exceptions of course where you see flaws at 1024x768 that arent there at 8x6, or vice versa. ANd I never tested PAL discs (Though I do have a few someplace) because they had to be ordered from europe, and mostly I bought locally and stuck with POPULAR titles and new releases, so waiting an extra 10 days to get em from UK vs day one at BestBuy didnt make any sense. So Ill make no claims to resolution visibility on discs made for PAL viewing on PAL equipment(WHich Ive never owned). PAL isnt just a res difference, its speed/FPS as well, whole load of small general alterations.
DVD GENIE will do auto resolution switching on disc insertion if I remember. (You configure it to be the loaded program on DVD detection, then configure IT to fire up your player of choice at whatever resolution you specify...this is memory thats like 3 years out of date, but it used to work like that if I recall) WWW.INMATRIX.COM is where you can get DVD GENIE and quite a few other tools as well. (All free of course).
Like I say, MY MEMORY is, that me and half a dozen outside people brought in to choose WHICH ONE IS SHARPER couldnt consistently (Or even regularly) choose the higher res over the lower res image at equal window sizing or full screen testing. (Or choose a 10 pixel 5% color flaw at 800x600). So in all likelihood, as long as you choose say 1024x768 for the loading resolution of your DVD player software, You will see it all with little loss on lower res discs...at least little youd be able to see IF any.