In my nonprofessional opinion, you need not charge your customers sales tax.
Sales tax is to be charged on the "selling price of tangible personal
property and certain services purchased at retail, i.e. by consumers."
Section 82.04.050 of the Revised Code of Washington (RCW) spells out
what transactions are considered retail sales and therefore subject to
the tax. Paragraph (6) says:
"The term shall also include the sale of prewritten computer software
other than a sale to a person who presents a resale certificate under
RCW 82.04.470, regardless of the method of delivery to the end user,
but shall not include custom software or the customization of
prewritten computer software."
"The term" here refers to "the term 'sale at retail' or 'retail sale'"
in paragraph (3). So assuming you are providing custom software or
the customization of prewritten software, your services are not
subject to sales tax. (Of course, if you sell, say, Windows XP in a
retail box, that is another story.)
The chapter of the RCW that deals with retail sales tax is chapter
82.08. Paragraph (6) of section 82.08.010 on Definitions refers to
chapter 82.04, cited above, for the definitions of "sale at retail"
and "retail sale."
I can't give you an opinion on local sales tax without knowing the
locality involved, but I think it is unlikely to be applied in a way
that is inconsistent with the state sales tax.
Washington State Department of Revenue Tax Reference Manual 2005
The section of this Tax Reference Manual on retail sales tax lists
"customized computer software" under "EXAMPLES OF EXEMPTIONS - OTHER
ITEMS," meaning it is not taxable.
The Department of Revenue's main page on the Retail Sales Tax has a
list of services that are taxable. None of them looks similar to what
Google Answers does not provide professional legal or tax advice.
I hope this is a satisfactory answer to your question. If you need
any further information about this, please ask for a clarification.