How consistent and free rolling your trucks need to be depends on how sharp your curves are, how steep your grades are, and most importantly, how long your trains are.
It sounds like you are in N scale. To avoid stringlining with long trains, a decent curve radius and reasonably free-rolling cars are critical.
Cars should roll freely down a less than 2% grade - which is the end of an 8" piece of track raised about 5/32". Or raise the end of the 8" track 1/8" which gives you a 1.6% grade. Any car which will not roll down the grade by itself isn't free-rolling enough and/or is too light. Very free-rolling cars will roll on less than a .75% grade - the end of your 8" track raised 1/16"!
The above would be my recommendations. Being in HO/HOn3 myself, I test with a 24" piece of track with the end raised 1/2" above level, which is close to a 2.1% grade.
Higher friction, especially in N, will significantly the number of cars your engine can pull.
The disadvantage of free rolling cars is the Kadee/MicroTrains coupling "dance" and "slinky effect". Light cars that are very free rolling are difficult to couple when pushed together. The scissors and spring design of HOn3 and N couplers can give a longer train of free rolling cars a "slinky" look as cars move closer together and further apart as they roll along.
my thoughts, your choices