Altitude data from any GPS is not particularly accurate, due to the way that the GPS system works. I'm not sure if WAAS and similar things help with this or not.

Many GPSs now have built in barometric altimeters that are quite precise but not accurate (the two words mean different things in scientific terms) -- they can detect that you've gone up a foot, but changes in the weather will make them be off by quite a bit over periods like an hour or more.

The Garmin Edge 305 and 705 have this barometer. The 205 and 605 do not. I imagine the newer models have it as well, but the cheaper ones may not.

I imagine that the GPS "calibrates" it's internal altitude via the GPS when it gets its initial fix, and then uses the barometer to detect changes. This is ideal for capturing the slope of a hill, as the high precision works perfectly for this, but it can also mean that by the time your ride is over and you're back to your starting point that it thinks you've gone up 100 feet because the weather has changed.

In any event, if you have one of these GPSs, as long as you have a dozen or more feet of altitude difference between the points where you want to calculate the grade of that region and the data points were taken close to each other (seconds, minutes apart -- not hours) ... it should be pretty accurate.