For cumulative altitude gain, Ascent is the most accurate software I've found to date. It usually gets within 1 or 2% of the actual gain on long climbs.

The cumulative number that the Garmin 305 reports is usually too high, so I don't even look at it any more.

I used Polar for several years, and it was consistently 5-8% low. It suffered from barometric drift, especially on large vertical gains. While the Garmin uses barometric measurements, it seems to use the GPS readings to recalibrate the altitude in real time. When I get to the summit of a climb, it's almost always dead on.