ALL spark plugs need to be gapped according to manufacturer specs. If you do not it will affect the overall timing. Larger the gap the longer it takes for the spark to jump and conversely for a closer gap is takes less time for the spark to jump.
That is not entirely true. Some plugs have no way to be gapped. An example would be of the plugs found in some rotary engines in the RX's. As stated above, if you are going to gap them and check the gap, use the right tool. Do not use the old round tapered gapper or you can ruin a new plug. I have found most factory plugs are already pre-gapped correctly.