I have about a 250' run, and have never used an amp. I've tried them, but they didn't improve anything. I've been working on moving my dish out another 50-75' or so, but before doing that, I wanted to do some tests to make sure that I didn't degrade the signal too much, so today I took out a spool of about 75'-100' (not sure which) of coax, and inserted it between my motor and the LNBF. It did drop the signal quite a bit, but didn't hurt the quality all too much. But then, I decided to try my inline amp again. This time it finally helped. Brought the S/Q almost up to where it was at it's best before.
So anyway, my opinion is that up to 250' you don't need any amp, but if you get up in the 325-350' range, then yes, an amp will help.
I'm hoping to move my 3' dish up on top of a new garage we just built last fall. I think from there I should have a much better view of the sky, but I was afraid that the extra 75' might make things worse, but maybe not.