Hi --
It may sound nuts, but the slop in the score is what makes the music sound interesting.
Some MIDI files contain quantized notes, that is, notes that were corrected to a timing grid. This makes for a good looking score, but robotic-sounding music. Of course, your mileage may vary...
I don't think there is any way to know in advance if a MIDI file will display neatly or not. Personally, I plan to buy a score from a vendor like Sheet Music Direct and not depend upon a MIDI file itself. Yep, it costs more. Sometimes, one can find a score or lead sheet on-line for free.
Hope this helps -- pj