RSickles is right on. For steel bearings (OEM, Advantage, Avid, Boca, Acer, etc.) anywhere except the clutch, keep the grease in. The advantage you'd derive from degreasing and oiling those bearings is quite small, and once you do so, you really need to pull the seals from the bearings after every race day and re-oil. Steel bearings stay greased for a long time (months!)...but when oiled, requires regular maintenance.

For clutch bearings, you can keep the grease in for longevity -- however, some of the grease will fling into the inside of the clutch bell, causing the shoes to slip and the bell to get very hot, which is a risk for bearing, shoe, and spring life. Many de-grease their clutch bell bearings (steel), and add a drop or two or bearing oil instead. This causes minimal fling into the bell, but requires much more maintenance.

IF you're going to oil instead of grease your non-clutch bearings, I highly recommend TS-1+ lube from TrackStar R/C. We've tried almost a dozen bearing lubes recently (actually looking for one to OEM and sell under our brand), and we've recently made the decision to use TS-1+. It's an outstanding custom mixed bearing lube. Many of the other bearing oils caused or allowed rust in the bearings...this one doesn't. Good stuff.

Ceramics are a different story. The balls are designed such that they stay lubricated much longer with oil, and are designed to be used and maintained with oil (NOT grease!). They do require more maintenance than steel bearings (recommend checking, cleaning, and re-oiling every 2-3 race days), but not as much as steel bearings that have been oiled.