No the widget is not really necessary as both Technorati and Google spiders recognize wordpress.com categories as tags. Be sure to use categories on your posts.
You cannot change the font on your header without a knowledge of CSS and the purchase of a css customization upgrade. However, if you have a theme that allows for a customized header than you can use different font in the header and upload it and in that case there is an option to "hide text". http://faq.wordpress.com/2006/07/08/which-themes-can-i-add-a-header-image-to/
Fonts can be changed on posts without the css upgrade http://faq.wordpress.com/2006/06/12/can-i-change-my-font-font-size/