I do not believe the guides on regular tours are allowed to accept tips. However you can offer and if declined, realize if you keep offering, they will accept, however it will go to the Children's Hospital of Orange County. (They accept after the 3 decline, turn to their lead, and it is donated.)
We tipped our guide on the Halloween tour. I asked her if she was allowed to accept tips, she said yes but it wasn't necessary but I tipped her anyways.
My understanding is that they have to decline three times before accepting a tip if you want to tip them. I've been on tours and my friend has tipped the guides.