Some customers also look at the address bar to see if they are on an https site. I know I do.

So, it's pretty easy to change the customer login URL to https

in includes\templates\your-template\sideboxes\tpl_shopping_cart.php find the line

<a href="<?=HTTP_SERVER.DIR_WS_CATALOG?>index.php?main_page=login">Login</a>

and change the HTTP to HTTPS

<a href="<?=HTTPS_SERVER.DIR_WS_CATALOG?>index.php?main_page=login">Login</a>