Let modify the node even when previewing it

What if one wants to automatically strip tags from the node title submitted by the users, when they click on 'Preview'?

In hook_nodeapi(), 'presave' can modify the title just before the node is going to be submitted, but not when the node is going to be previewed.

I know it's considered bad style modifying the data when validating.
But I'd like to automatically modify the node title even when previewing the node itself.
I don't want users have to modify it manually (IOW, I don't want to use form_set_error()).

I think I could also do it using hook_nodeapi() when $op is 'view' (and 'alter'), but this means my code will be ran each time a node is viewed, and that's not good: I'd like to run it only when the user is going to preview/insert/update the node.

I really think it should be inserted something like a $op == 'preview' action to hook_nodeapi():

Am I wrong?

Greetings, Giovanni