Let me complicate things a bit - because that is what I do
Normally, send your paycheck contributions to C/S/I or chosen L Fund. That way your new assets start working for you right away. When the equity market funds are dropping (as they are now) contribute more. When they are rising still contribute to C/S/I/L. When they are toppy you might want to start contributing to G/F. Kinda pointless if you have a good chunk of mullah in your account, but...
To me, there is no intrinsic value in sending new money to the 'G Fund'. Take last year as an example. Say you are throwing $300 into TSP (including match) each pay period. The market grinds up by 2% a month or so. You sit that money in the 'G Fund' till you have $5,000 and then invest it in F/C/S/I. That scenario leaves your first contribution in a cash account for 16 pay periods earning nada. Zip, zilch, nada for seven or eight months. What a waste. That $300 would have been worth about $350 by the time you made your IFT into the equity funds. Now last year was a bit of an aberration - but the thought still holds
But, I can hear Amoeba squeaking in the back. What would have happened in 2008 if you 'invested' your $300 contributions to C/S/I. You would be broke, broke, broke!!! Market crash. The end of the world. You would be fighting the dogs at the feeding bowl - but we know you would be the Big Dog so all would be alright

. Anyway, here goes. You put that $300 into G from October 2007 on. In April 2009 you got yourself $5,000 to invest at almost the bottom of the market. The 'C Fund' collapsed 57%. Had you been putting the cash in the 'C Fund' you would probably have about $3,500 - $3,750 (the math would be nasty and time consuming - each $300 contribution would have been cut by a smaller percentage as the pay periods go by). Let us use $3,500. You save $1,500 in retirement savings - negligible when talking retirement. But, what exactly isn't negligible is the fact that in April of 2009 you would be sitting with $5,000 in cash via contributions, probably all cash in the account, and sweating the panic. If you are not
very disciplined you will not IFT everything to the equity funds in April. You will sweat in a dark corner saving electricity and eating macaroni. Tell me that isn't the truth. Show me on AutoTracker all those who made IFTs in March and April 2009 to the equity funds. I'm not doing all that work, but by looking at the final tallies I can tell you that out of those who made 20%+ in 2009 (a year the C Fund grew by 27%) twenty of the seventy money makers made no trades - they rode it down and rode it back up. Fifty made trades and only forty three made more money that the 'C Fund'. The other two hundred and eighteen folks DID NOT buy low. A lot of them were sweating in a dark room. Are you one of the Warren Buffet's around here?
I'm not saying that you - or anyone around here - cannot do a bit of market timing or allocation management, but I am making a point that playing with contributions is kinda pointless. Send all contributions to C/S/I, play with the account balance via IFTs. You want to take every opportunity available for buying the 'C Fund' at $7 bucks you possibly can. Who cares about $1,500 one way or the other when your retirement account should be in the seven figures in your golden years. More important than the $1,500 bucks is time in the market. Miss years of equity growth and you had better not own a large dog because they like to eat to - maybe you