There are two forces at work here:
1. Yes, if you contribute the money up front, it will have longer to grow.
2. If you are investing in stocks, you are better off dollar cost averaging by investing monthly than hoping the first half of the year is a good time to buy.
If it's possible for you to put all the money into a money market fund up front but can also internally transfer it gradually to a stock fund, then you can have the best of both worlds.
If you're primarily investing in a money market or short term bond fund anyway, then you might as well get it in as quickly as you can.
Otherwise, go for 1/12 per month and dollar cost average. On stocks, the effect of dollar cost averaging outweighs the benefits of having a few months more to grow.
BTW, I wouldn't try to predict when the market will be low and when it will be high. Nobody can. Over time, dollar cost averagers win out.