If you got a slanted base you increase the value of your scope adjustments. Sometimes there is too much and you can't zero for closer distances. A friend had a 20 MOA base and with his scope turrets bottomed out he was still 7 inches high at 100 yards. He could not adjust it any lower.
If you get a scope that has a lot of travel, like the Falcon series or the Super Snipers for instance, you get over 100 minutes of travel.
It takes the average .308 approximately 40-45 minutes to get to 1000. If I mounted my scope and deviated from my scopes "mechanical zero" by 50 minutes of elevation to get zeroed at 100 yards, I still have enough travel left to get to 1000 yards without the use of a slanted base.
As for the TPS rings. Buy them and don't look back. They are awesome. Find out how much travel is in the Bushnel Tactical you purchased and see if you need a slanted base for the distances you may shoot.