If you want to make things easier, you could enter your spoilers in a paste site. I made one for this purpose, which supports Less Wrong comment syntax for formatting:
Behold, yet another paste site!
Just enter some text, and then copy and paste the URL. For example:
That's pretty cool, but I wonder how long the site will exist. If it goes away, so do any spoilers stored there.
Every time I come across one, it annoys me. I have to copy the text, open a new tab to the ROT13 site, paste the text, and click the translate button.
Compare this to something like a collapsable spoiler button-box where you press a button and it expands and expands a box with the appropriate text underneath it. Even making a [spoiler][/spoiler] tag that gave a black background and equally black text would be better than the current ROT13 solution.
Was there actually a reason for doing things this way? If so, why not just include the ROT13 translation in the javascript that'd open and close the textbox? Comments? Criticism?