If we sin against a brother, were supposed to make amends with them, so I dont se why sinning against God would be any different.
With respect to our salvation- personally I think sin that goes unaddressed in prayer can harden your heart- even if its sin that we overlook and dont know about.
When you pray, pray in this fashion
Our Father in heaven, hallowed be thy name.... you know the rest. Forgive our trespasses as we forgive trespasses against us...
Right? Were supposed to be asking the lord to show us what were doing wrong and help us fix it. I think not doing that can gradually kill your faith. We're supposed to be using our heavenly treasure- faith, and understanding- but if we don't employ that which we have, it can get taken away, and we end up like the servant that gets thrown into the outer darkness.