If you do not have any money in pre-tax IRAs, you can just do a backdoor Roth IRA contribution -- contribute to Traditional IRA and then convert to Roth IRA -- regardless of how much your income will be this year.
If you do a Roth IRA contribution now and later find out you're over the limit before the tax due date, and you want to still have the money in Roth IRA, as littleadv and Dilip Sarwate mentioned, you can either a) re-characterize the Roth IRA contribution as a Traditional IRA contribution, and then convert to Roth IRA (basically, replicating a backdoor Roth IRA contribution, but with a long delay between contribution and conversion); or b) withdraw the contribution + earnings out, and then make a backdoor Roth IRA contribution from scratch.
But either of these ways will cause you to pay taxes on the earnings from when you contributed to when you rectified it: with (a), you pay taxes on earnings on conversion; and with (b), you pay taxes on the earnings when you withdraw the contributions before tax due date.
On the other hand, if you had done a backdoor Roth IRA contribution to begin with, you don't pay taxes on these earnings.
And (assuming you have no money in pre-tax IRAs), a backdoor Roth IRA contribution is essentially identical to a regular Roth IRA contribution, except there's no income limit. There is no downside to it. So even if there's a remote chance that you'll go over the limit, I would suggest going with that.