Are we talking here about finishing fermenting in secondary fermenter or are we talking about transferring fully fermented beer to another vessel for flocculating and clarification? In my case, I consider it to be the latter.
These are two different things.
What the rest of these guys said... Technically, going to secondary is done either at partial or full fermentation. Ten years (or longer) ago it was pretty common wisdom to rack to secondary when fermentation was about 0 - 80% complete - often about five - seven days into fermentation. Doing so prior to completion of fermentation left a lot of yeast in suspension to help ensure completing the fermentation in secondary, while getting the wort off of the trub. The idea was to allow the beer to bulk mature and clear prior to bottling, to create cleaner-tasting beer while removing the threat of autolysis. Dave Miller, in "Brewing the World's Greatest Beers" (1992) recommended racking off the trub 12 hours after pitching the yeast.
Since then, fears of autolysis have subsided significantly, and kegging is more commonplace. The same bulk maturing, and/or cold conditioning happens in the keg, so arguably that IS secondarying. Or, if people are more particular about getting any yeast at all in their keg, or want to bulk age, dry hop, and/or add fruit prior to bottling/kegging, they will secondary in a carboy, conical, etc.
Personally, I learned a long time ago I prefer to secondary in carboys only after fermentation is done, and normally if adding fruit, or want to dry hop or cold condition, or see the when the beer has cleared. And I recently went to kegging about 1/2 of my beer and so use the keg to secondary/dry hop/cold condition.
And sometimes I'll still go from primary to bottling bucket, especially for a low gravity ale.