Is it necessary to use the trademark symbol throughout a website?
Use of the trademark symbol is optional, but it doesn't hurt.
For logos it's often part of the image file, so it would actually
be more work not to use it.
This page is full of reasonable examples. In the banner we have
the Answers logo with TM, and at the very bottom we have a
copyright notification, but for running text including the word
"Answers" (such as the Answers Properties list header and content),
there are no markings.
Other websites are almost cartoonishly consistent with their use
of the symbol--every logo, every slogan, every time the name of the
company or its products appear, there's a helpful (r) reminder in
case you'd forgotten in the intervening half-inch that they've
registered it all. These are cases where using the indicia is
distracting.
What you want is a balance.
