G'day Alex,

You're right: this is a minor annoyance. I usually just preview without the 'spoiler' tags (or placeholders for them). When I'm happy with the post, I add the tags and preview again.

When previewing a post with 'readmore' tags, the markup looks like this:

<div class="readmore">
...
</div>
[download]

Perhaps something similar (with class="spoiler") would be appropriate for 'spoiler' tags.