I don't rebalance generally unless the opportunity is (in my opinion) large enough to warrant realizing capital gains. I tend to believe Bitcoin does better than alts when it is moving rapidly in either direction. Consolidation is what kills Bitcoin's dominance. If we see another bull run like we did in beginning of December, I suspect we will see as you expect (altcoins moving to Bitcoin). But I also think the opposite is true too: If we see a bear run for Bitcoin, then I also think altcoins will flee to Bitcoin.