I think that Tony considered Paulie to be fiercely loyal to him,and willing to follow his orders regardless of the consequences.

Even though Tony was a very smart,street wise Boss,
he vastly underestimated Paulie. Part of it was Tony's ego. He felt that Paulie was basically stupid,and that without Tony,he couldn't make it in the Mob world.

As we saw,Paulie was treacherous enough to try to ingratiate himself with Carmine Luppertazzi. In his mind,after Tony was gone,Carmine would support him as the new Soprano Boss.

When his plan fell apart,he must have been quaking in his boots,waiting for a bullet in the head.
The fact that Tony never found out is very unusual,given the nature of the Mob grapevine.

Remember,Paulie almost did get whacked by Tony,when they were on the boat in Florida. If Paulie would have been a little less cautious,and maybe more drunk,he might have figured,"What the heck,Tony's my best friend,and the Ginny Sack thing was so long ago,I'll just come clean.We can both have a laugh,and it'll be forgotten.

Lucky for him,Paulie was so paranoid and distrustful,that his gut told him not to admit anything. Had he copped out to Tony,Paulie might never have left that boat alive.