Difficult subject. Your stock focus screen only allows precision equivalent to an aperture of about f/4. This is far inferior to what you can manage with live view. In fact, live view will beat out even the best screens. If you pixel peep (equivalent to live view), f/5.6 at 20 feet won't make any difference if focus is actually missed. Ditto for f/16. DOF only buys you a slop factor for a particular degree of magnification of the final image. It does not change the actual point of focus.
For manual focus using the viewfinder, an aftermarket screen is essential. The Canon S screen mentioned above is a good option. I use a Katz Eye screen and find that the focus aids (split-image and microprism) make all the difference when focus is critical. Before the Katz Eye, manual focus was a exercise in frustration.
A few notes regarding metering and AF and aftermarket screens...
- All aftermarket screens are 100% compatible with the AF on your SLR. The AF system uses a completely separate optical path.
- Screens with focus aids may affect spot metering, but the other metering modes should be unaffected. Assuming, of course, that your camera supports other than center-weighted metering with your manual focus lenses!
In regards to catch-in-focus...it is essentially the same as using the green hexagon for focus confirm. Both are prone to the same issues with a manual focus lens as with an AF lens using same system.
Steve