Two thoughts for you:
1) If you're investing in a target date fund like that, you're presumably doing it because you like their AA, so stick with it, no rebalancing needed.
2) If you do want to tweak your AA, rebalancing during the accumulation phase is best done by sending contributions to the underweighted assets, rather than selling overweighted ones (unless you're WAY out of balance, but that doesn't seem likely most of the time).