Is it really necessary to O-ring the block when its sleeve to bite down the headgasket more, or nobody uses it they just go with a thicker headgasket?

 

Not at the pressures that most people will be using on the street. (even low 20s)
Be sure the block and head are properly surfaced. (trued and smooth without machining marks) Stress caused by O-ring material interrupting the even mating of the head and block surfaces are a concern. Some still think O-ringing is best for higher pressures.