I guess it depends on philosophy. But the sooner your money is invested the sooner it can start growing. The flip side is in the short term you could get real unlucky and the market could crash the day after you put in a lump sum. In which case your investment would have bought you a lot more had you waited.
But that gets you down the dark road of market timing, which can lead you to stagnate investments when you shouldn't for fear of market drops. So holding that strategy may just as easily lose you a tone of money in the future.
I am getting ready to fund half my 401k for the year in one shot. I hear people saying that the US equities are likely to take a hit in the form of a correction soon, which would suck because a good 40%-60% of my investments are in US equities. But it makes sense because they have had a steady growth over the last 3 years.
To assuage this fear, I am first pretending I am not dumping a large chunk of money in and only caring about the asset allocation once its there. The correct response to a potentially over-performing asset, from my limited reading, is to re-balance your portfolio to shift money away from your over-performing investments like US equities into investments that have been under performing like international stocks. I am making a 10% re balance this year. This is just buying low and selling high. I am selling some of my high gaining investments to buy up some under performing investments for less than they will cost me in the future. I am re-balancing from 55%-60% US stock investment to a 45%-50% US stock investment and increasing my foreign allocations from 30%-35% up 40%-45%.
This is the gist of the simple strategies I have read from index investing. But I am also young so if anyone has any better advice I am listening.