It is never a bad thing to add fluid and top the level off. It is however a very bad thing to add fluid and overfill the transmission. Park the vehicle on a level surface and check the fluid level according to the instructions in your owner's manual. Add only enough of the correct A/T fluid to fill it to exactly the full mark. It takes very little fluid to top it off. On most vehicles when it is at the add mark it will only take one pint to top it off. Just be careful and do not overfill.
Clay