Magento did not mention this (or hide somewhere I could not find), so I’d like to mention it:
Magento helper class is a singleton desin pattern. Whether a helper class is called in a format as
Mage::helper('mymodule')->helperMethod();
or as (in a block class or a template),
$this->helper('mymodule')->helperMethod();
the helper class is only initialised once.
Which means Magento puts a singleton desin pattern on helpers.