Rent Dispute, need advice

I'm trying to find the common practice in regards to how rent is paid when leaving a property in mid-month. This is a dispute amongst roommates. When there is no lease or renters agreement, is rent always paid for a full month even when the renter leaves before the 1st, or is the rent pro-rated? This is in San Diego, CA, if region of the country matters. Here is the story to understand:

At the beginning of April 2008, the plan was for all 5 roommates to find a new place of residence. Only 1 of the 5 roommates is on the lease with the property management company, all other roommates are paying month to month with no formal written agreements (at least not for myself and the roommate I'm moving away with).

There was a discussion in April that everyone would be out on July 1st. After a few weeks, 3 of the roommates decided they wanted to stay at the current property. Myself and the remaining roommate found a new place of residence. We told them in the middle of May that we'd be gone by the 10th of June. It may not have been exactly 30 days but close. I don't know if we even need to give them 30 days notice if only as a courtesy.

They have now told us they expect the two of us to pay our full portion of the rent for the month of June. They are saying that we all agreed to stay until the 1st of July from the discussion in April. My understanding of that discussion was that we'd all be gone by, or before, the 1st of July. Since we'd already told them we were out on the 10th of June I planned to pay a pro-rated amount of my portion of the rent for those 10 days.

Any thoughts? Thank you so much in advance.

That really is by agreement.

There does not exist the right to just take off normally.

What was the agreement when each renter came in? That they would stay until what date?