The tee, or ell, had to point downwards. Turning it sideways would have done nothing to make it work properly. In addition there should have been a section of pipe, about 2' long glued into the elbow. The problem you can have with an elbow, (but since you do not have the pipe, it probably will not be a problem), is if soap and grease float on top of the water inside the baffle you made and harden. With a tee you can break it up with a stick, assuming you also installed a cleanout to ground level in the small access port. Without the tee, the cleanout would have no function and you will have to snake to the tank from the nearest cleanout, which can add to the expense if you do not have the proper snake to do it yourself.