betting spread..

What is the lowest betting spread that would still be effective?
I was curious about the 1-3.

In my hometown casinos, I would either be playing 6d H17, DAS, double on any 2 cards, not sure on the penetration. Or dd with the same rules. The 6d is $5 minimum and the dd is $10 min. off peak.

Also, what is the general rule for a session bankroll?
Thanks