It's not necessary, especially if you are fairly confident that you will be accepted into your 1st/2nd choice uni. Travelling to open days can be time consuming and expensive so don't feel pressured to go to any more than is necessary. If for whatever reason you do end up deciding to go to your 3rd/4th/5th choice university there are sometimes post-offer open days, or you can always just pop into the university at any time to look around and get a feel for the place during a normal day.