I tried a tap I know is the right one for water bottle bosses (M5x0.8) in
some fender eyes on a better quality bike and it is indeed the same.
However, I see cheaper bikes with punched holes and another with M6 holes.
You can be sure that there will always be some tap you find you need and
don't have and you'll spend a lot of money buying them piecemeal. For $17
you can buy a 60 piece SAE/Metric tap and die set from Harbor Freight
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=45766
which includes the handles and thread gauges. These are not toolmaker
grade, but for the most part you'd only be using them to chase painted or
damaged threads.
John Betmanis <johnb@oxford.net> Woodstock, ON Canada