It all depends on what you're shooting and what your personal preference is for that shot.
Most landscape shots won't have many things drastically out of focus at sensible apertures (unless something is very close in the foreground), so without a tilt shift you can argue in some cases it's worth sacrificing ultimate depth of field to avoid diffraction softness, although equally you could argue it the other way around.
However, with a much narrower depth of field such as macro, if the depth of field is too narrow, it'll be painfully obvious even in a thumbnail sized image. Stop that lens down loads, and it'll look much better overall to everyone except pixel peepers.
In summary, do what suits you. But if you want to be as close to perfection as is possible, start saving for a TS-E lens.
There are a few problems with TS-E lenses, mostly related to them being quite a faf in the field. Complex movements need to be mastered and they really are quite hard to master at first and are very hard to set up in changing light. There's filter issues with the wider TS-e lenses, which can be remedied but only add to the complexity of use. The new Photodios filters come to mind...I have a set. There's also the fact that these are primes and loose versatility to any zoom. It's quite likely that a photographer can't get the desired angle of view with just one TS-e lens. Then the photographer needs to lug about all four TS-e lenses for every eventuallity. They are heavy for what they do and four of them is a lot more heavy than a bag of f2.8 zooms....in my opinion, TS-E lenses are not practical for general landscape work and certainly not practical for low light landscape work. Sure they might be good for mid day work - architectural stuff, but for windy sunsets on the coast....er....no. I'd choose diffraction over a TS-e headache any day.
Don't get me wrong, I really like my TS-e lenses in the right contex. But for many applications, a zoom is a better choice.