Let's take a classic example. An instrument rated pilot is flying a C-172 and wants to maintain instrument currency. The pilot goes flying and performs some approaches, holding and intercept/tracking to maintain that currency, all while under the hood. In order to maintain a safe condition, a safety pilot is brought along to maintain VFR conditions.

The question is, how should the safety pilot log this time? In this condition the safety pilot is a required crew member, but is this time technically SIC or is it PIC?