It all really depends on the plane. You'll make life easier on yourself by trying to stay out of your own way. While you are at it, think about maintenance- the fuel tank will need to come out at least annually for new lines and a new stopper, and the battery will need to be replaced periodically, etc. Like J.Duncker, I usually install the battery last as it's an easy way to move the CG around. If I think I might need to put it under the fuel tank, I wait until I've checked the CG before installing it.