Should I contribute to a Traditional IRA now?

I expect my 2018 MAGI will be somewhere around $110,000 (MFJ). That's definitely an estimate, but it will most likely be somewhere in the middle of the Traditional IRA phaseout. I plan to contribute as much as I can to the Traditional and the rest to Roth. Am I able to contribute now, or do I have wait until I file my 2018 return and get an exact MAGI? Even if I can, is it smarter to wait until I know so I don't over-contribute? I hate to hold the money in cash all year and lose out on the earnings, but I don't want to run into trouble if my estimates are off. Any advice would be appreciated!