It kind of depends -- the problem is, if you can easily mentally edit the watermark out using the human eye/brain combination then you can probably physically edit it out easily using Photoshop.
The ways to make a watermark more difficult to remove through scanning and Photoshop are (a) put it on top of important information and/or (b) make it more obtrusive (i.e. destroy more of the information under it).
For example, in these three shots watermark #1 would be pretty easy to remove, watermark #2 more difficult (covers important information) and there is no way, even if I were so inclined, that I would attempt to remove watermark #3.
But as a tradeoff for this, it is much easier to get an impression of the original image with watermark #1, more difficult with #2, and much more difficult with #3.
As with so much in life, you have to balance two competing interests and decide which is more important to you.
[Note to anyone else reading this thread: Sue Chastain's tutorial (non-
RP) can be found
here ]