Ride where you're most comfortable.
That said, I ride with the front tire just to the left of the lane center line. Most roads, if they are well used, have 2 slight valleys or wheel ruts where 4 wheelers wear or compress the road surface. If you ride a bit to the left of the center line of the lane your front tire is running on the side of the left wheel rut, being pulled slightly back to the left against the crown of the road, cancelling out the pull to the right. Again, as roads vary, how much off the center will depend on the original crown of the road and how much the road is worn by cars and trucks.
You'll have plenty of room - you needn't ride much off center and your trike won't be too close to the middle of the road, though I generally like to ride with the outside of my fender just a few inches from the center line stripes. Riding to the left also gives me somewhere to go if an oncoming vehicle strays into my lane, and may let me see a few more feet around a curve. Seeing that few feet can make a big difference in getting hit or missed by an inch.