You do not have to wait until the end of the rescission period to begin accruing interest. This is from the Reg Z OSC:
"3. Actions during the delay period. Section 226.15(c) does not
prevent the creditor from taking other steps during the delay, short of
beginning actual performance. Unless otherwise prohibited, such as by
state law, the creditor may, for example:
Prepare the cash advance check.
Perfect the security interest.
Accrue finance charges during the delay period ."
Opinions expressed are mine and not necessarily those of my employer.