Over the weekend, I tried various combinations of 480i, 480p, 720p and 1080i output by my 622 to my 720p LCD.
My conclusion was - although the TV has a very good MTK scaling chip, nevertheless in every case, 720p output by the 622 looked better. This was on 1080i and SD channels, as well as 720p ones.
My LCD is probably typical of the average TV that people are buying, so I think that most people will probably get best results having the 622 output the display's native resolution, rather than passing through the channel without scaling.
This may have something to do with Dish's use of non standard resolutions, which mean that the 622 will always be doing some scaling (except in those few cases where exactly 480i, 720p, or 1080i is uplinked, which is evidently a minority of channels).
My results seem to indicate that either the 622 is scaling and the TV is scaling, or else just the 622 is scaling, and that the latter is better because of a smaller quantity of conversions.
Of course, YMMV, results may be different with different displays.
PS Note that the above is true even with 480i, ie SD channels looked better when the 622 output in my display's native resolution of 720p.