I'm not sure I understand the question.
Some template designers will not allow others on their own to modify their templates, but will produce modified or custom templates upon request -- for a price.
It all depends on what the template designer has chosen to include in their licensing terms.
If, instead, you're asking if it's OK to use an unmodified template for a website, or if you must modify it for it to be acceptable: you can certainly use unmodified templates "as-is" if they meet your design criteria. There is no need to modify them unless some element of the design is not to your taste or doesn't conform to your company's or client's design standards.
Diane Aull - NineYards.com
: Helping Businesses Do Business Online
Whether you think you can, or that you can't, you are usually right.