I went through this a while back for my own home. The story I got from both Dish and DTV was that their standard installers would not touch anything more than something like 60 feet. Anything more than that and you had to deal with their "pro" dealers.

As noted, the main issue is the voltage drop that signals polarization change at the dish and to power the LNB. The voltage switches between 18 volts and 12.  If you have too much voltage drop, then the 18 volt signal looks like 12 and you have trouble.

If you can manage the extra cost, RG-11 is a better choice due to its resistance being half as much as RG-6. At 300 feet, RG-6 voltage drop is around 2.5 volts (cutting your margin almost in half). Whereas for RG-11 it is 1.0 volt. 

Here is a little spreadsheet I created comparing the two cables showing both voltage loss and signal:

LNB Voltage:18
LNB Current :0.3
Distance :300ft

Type: 5787 CommScope(RG6) / 5917 CommScope (RG-11)
Loss/ft: 0.029 / 0.012
Resistance Total: 8.580 / 3.600
Voltage Drop: 2.574 / 1.080

Loss per ft@2200Mhz: 0.097 / 0.054
Total loss @2200Mhz (db): 29.100 / 16.080