I don't think a thank you note is warranted. There are house tours in my area all the time and although I've never gone to one, I'm just not seeing sending a thank you.

Then again, you can never really go wrong with a thank you. I'm just not sure who your thank you should be directed to -- the home owners, the historical society, the person who organized the event-- all of them?

I'm interested to see what others have to say...

As you paid to attend, I don't think a formal thankyou note is required. You don't send thankyous to the cinema, concert hall, bowling alley, etc, after all.
The people who should be writing the thankyou notes are the tour organisers, who were graciously allowed to use the historic homes for a fundraising event.

A thank you card is not required given you paid to attend. I'm sure the lovely tea was financed by the fee and therefore it is not necessary to thank anyone.

However, I see no problem with writing a little note to say how much you enjoyed their gardens and how nice it was of them to open them for you to enjoy. It's a lot of work to get a garden tour ready...a lot!

Not required. One would be appreciated I am certain if you wanted to send. Just to know that someone enjoyed the hard work that the hosts did. I'm sure dealing with tours are not easy.

But, again not required!

Same here: not required, but a lovely thing to do, and the recipients will be delighted, especially if you comment on something you noticed in their homes.