1. John had managed to work his way into a position of power and influence. He was certainly sitting pretty until it was discovered that he was stealing money from the company and was fired.
2. I’m going to be careful how I spend and save my money now, so that when I retire I’ll be sitting pretty and won’t have to worry about my finances.

The expression is often used to describe a financially advantageous situation (opt.2). It is always used in the present participle form. It can be used in the past or future tenses by making the verb be past (opt.1) or future (opt.2).