You gotta ride in the middle so as not to "invite" a car to encroach you.
With respect to rain and the OP's inquiry, it's going to bring the oil and fluids out of the middle road, no bueno unless it's been steady raining for a while. Even on a good day, the middle is still holding the fluids and the regular track of vehicles is the best choice for me, left or right 1/3 of the lane depending on how I want to position myself for my benefit.
For lane positioning on good days, the left 1/3 puts a rider in the most advantageous position for visibility to on coming traffic.
When riding solo on two lane, I use the left 1/3. Pretty much a standard and established preferred position in riding. As oncoming traffic becomes visible, I'll move over to the right 1/3 to give myself a better view of what is behind oncoming traffic (being aware of other vehicles behind me and taking that into account). This also allows drivers following the oncoming car to see me better and lessen the potential for them pulling out to pass the car in front and I can see them making a bonehead move to pass sooner as well. Then move back to the left 1/3. Left 1/3 is the preferred position for me to see and react to deer, elk, bears, dogs, other drivers turning into the road from the right side, etc.
When riding with two or more, staggered is the choice and stay in my 1/3.
On divided or two or more lanes in one direction, left lane is my choice and it's a continual survey and positioning for what is in front, coming up from behind, anticipating the potential for drivers to do "the sweep" and cross multiple lanes, etc.