I use inflatable PFD's with harnesses. Here are my rules:
1. If you are more comfortable in any situation, where it and tie off. Being more safe is always a good option.
2. When I singlehand, I always where the PFD/harness and tie off when leaving the cockpit.
3. In protected waters (Galveston Bay) it is optional unless I say so (heavy weather or traffic).
4. In unprotected waters (Gulf of Mexico), always tie off if out of cockpit and always in PFD/harness when in the cockpit between sundown and sunset.
5. All of these rules are subject to change slightly based on the conditions, the experience of the crew, and good judgement.
This seems to work well for me, I always feel comfortable and the inflatable PFDs on the market today are fairly comfortable, so not really much of a hassle.