Riding in the highest gear all the time will mess up your knees.
The gear you select for a single speed should be right in the middle, so you can climb and also sprint.
You can experiment with your bike before converting to figure out the right one. I suggest use the
small chainring and pick a middle gear in the back.

Deciding not to change gears or coast is not the same as riding fixed. You don't have the same intimate
connection to the road. You wont be able to back-pedal either, which is the easiest way to make small
decelerations on the fixie.