DataBase Performance Issues with Indexes
Fill factor will leave free space on your index pages. If you know that updates on index keys will need aditional space, then it may be ok to leave that 10% free. But if there will be no changes on index keys, it is better to have no free space on index (remember that all pages will generate logical reads on querys, so More pages = More logical reads = slower execution).

If you change fill factor, immediatly you will be asked to do rebuild