tedster - 6:39 pm on Apr 10, 2013 (gmt 0)
That looks technically correct as long as the character string "tagid" doesn't appear in any other URLs except the ones you want to disallow crawling for. If you say "Disallow: /*?tagid" then including the "?" would limit the rule to just query string parameters - that might be even safer.
Another step you can take is to use your Webmaster Tools account to tell Google to ignore the "tagid" parameter. Look under the Configuration > URL Parameters section.