Where should I be entering custom HTML on a SharePoint page?
Should I be entering within a web part? Direct into SPD? Or using the "Edit Source" direct from within the page on the SharePoint site?
Or does it just make no difference?
If you want it to be easily maintain you should add a Content editor webpart to the page, and add the text in the webpart. (More here http://office.microsoft.com/en-us/sharepoint-foundation-help/using-the-content-editor-web-part-HA101794745.aspx)