Tipping is done where you can't offer more for extenuating circumstances,
e.g. fumigation and sanding floors while or nearly while the work for hire is
done. This is an extension from the engineering and architecture end.
If the artisan captures your regard, invite 'em to the party you'd have
pulled the architects to attend or share snax.
IMHE, it's polite to determine whether or not the artisan can multiply and
plan enough to measure for a carpet, at first. If they can't (won't,)
they're either a dangerous bicycle repairman
http://www.jnlk.com/msshy/ledbottm.htm
http://www.rt66.com/~neil/resume.html
(Down with mandatory +0 orcish helm!
http://www.citylightsnews.com/pacyclo.htm )
or an artist who wants to know when the specs stop.