A good rule of thumb is to say "yes" to anyone who seems even slightly interesting or attractive to you.
You come to speed dating events to meet other singles. Give them a second chance to become a friend or special person in your life.
Unless… you are the type "who can’t say no."
If you are the type of person who can’t think of a good way to end a dating relationship or string people along rather then hurt their feelings, say "no" on the spot by not choosing the optional singles.
Yes, you will miss some good people but you will not be caught in as many situations where you have to reject a nice person later.
Just remember that a date of 3 minutes or 7 minutes or 8 minutes is not enough time to form more than a first impression.