Is it still a good practice to use oil base primer,because of it's penetration, and then topcoat with latex paint on exterior trim?
Unless the wood is either cedar or redwood an acyclic latex primer will do just fine as with todays wood oil based primers do not do all that well. The key on new wood is to apply 2 coats of finish over the primer so as to get a longer lasting job. If you need to do any caulking do it after you prime as this way the caulking will last longer, and be sure to use an acrylic siliconized caulk.
Again.......I am an old paint guy.......oil primer on raw wood - topcoat (twice) with latex.........works best....