easiest answer is to say:  It depends…

if you are using a stock, as delivered simple press theme,  you should not have to modify the theme at all in order for the plugins to function…  if at some point you made your own sp theme to customize the layout or colors, then you may have to add template functions to theme for plugins that may have been released AFTER you create your own them… but as new plugins are released, we do try to update the sp themes to use that plugin template functions… of course, where we put the template functions may not be exactly where you want that data displayed…  if you want the data displayed by an sp plugin to display differently or in a different location, you will have to modify the sp theme…

you did mention the template tags plugin…  That plugin is not meant to be used within the sp theme… they template functions within that plugin are meant to be used elswhere, outside the forum, on your wp theme…  that is, on other pages or in widgets or sidebars or other…  since its outside the sp theme, if you want to use the template functions,  you will have to put the template functions or shortcodes where  you want them to display…