I think it depends how you stay... If you stay on your back leg than it will come automatically to jab and slip at the same time.
If you are on your front leg already than you must slip first because you can´t move and your jab will be much more shorter than the oponents.Second variationm is to jab while you are leeping forward. Than yes you must slip fast after that because you are going to land on a really bad place. One for shure this are not movements for beginers! If you box from the front leg i want to point out something for you. Watch some Tyson fights for when he is jabing to set the right hand. How he is shifting his weight on his right leg and slipping at the same time to the outside. And the most important how he gets in same possition as lets say Joe Louis for example when he is jabbing.