Your feet should be moving forward if your going forward, slipping or not is negligible. Slipping a punch does not require a step out, you should be able to step straight forward and slip a shot at the same time. A slip comes from the waist/shoulders, it does not require a lateral step. Allot of boxing coaches teach slipping by using pivots. In short, step straight forward and pivot your lead foot just as you would if you were throwing a left hook..... slip the jab to the outside (assuming both orthodox). You can do the same by pivoting the rear foot outward (slip inside the jab or outside cross). Use the pivot and if necessary a slight bend at the waist or shoulder turn...... My opinion, this is a much more effective way to slip shots then stepping \ or / to do so. It keeps you moving straight forward and keeps you "in the pocket", it makes the transition to countering easier and quicker off either hand.
Remember, the goal when slipping a punch is to make your opponent miss, but that does not mean you need him to miss by a foot, that only makes the transition to countering slower and gives him more time to recover. Make him miss, but ideally, just make him barely miss or miss by a little bit...... much quicker transitioning to a counter.
