This year I jumped on it and made my 2006 Roth IRA contribution on the first possible date: January 3, 2006. Fortunately, I had the money available to transfer from my money market account into my brokerage account. I'd done the research and had my buys scoped and staked. I set the limit orders and bought into two ETFs and one stock. The buys executed a few days later. Checking my brokerage account this week I found that the earnings on my 2006 Roth contribution to date are $846.85.
My timing for contributions in previous years ranged from March to May. If I'd waited until now to make my contribution, I wouldn't be seeing a tidy 20+% already. While the market is a fickle thing, I believe these picks will perform steadily. Bottom line: if you can afford it, contribute sooner rather than later.