Open registration if and only if you are ready for routine maintenance, creating cleaning schedules, applying spam filters, balancing resource usage, etc. Not to discourage, but for you to know what you really want. If server is for friends and family, just leave registration closed.
My recommendation, as someone hosting a public server is to first host the server for yourself, play for some time, join some rooms on other servers, then invite friends, then if you feel ready and willing open it to the public.
Matrix Authentication Service is good to have, as you can use next-gen Element X which is most polished app as for now.