Wiped on 20 April 2010
You don't have to be at the last evolution - you could keep them unevolved until level 100 and get the full benefit of your EVs - but of course, you won't get Dragonite stats on a Dragonair, because Dragonite is stronger.
Your EVs might not completely show until Level 100, though. Every pokemon with the effort ribbon will always reach their stat potential (depending on EVs and hidden IVs, ie, chance) by level 100, but you might not get consistent patterns of growth until then. Often the stats just seem to come together between level 90 and 100 and hey presto, you have a decent pokemon (unless you really ballsed up your EVs or you just got shocking luck).
