The semantics of the QUERY method change to a "conditional QUERY" if the request message includes an If-Modified-Since, If-Unmodified- Since, If-Match, If-None-Match, or If-Range header field ([RFCHTTP], Section 13). A conditional QUERY requests that the query be performed only under the circumstances described by the conditional header field(s). It is important to note, however, that such conditions are evaluated against the state of the target resource itself as opposed to the collected results of the search operation.

The last sentence could be read to indicate that the QUERY content isn't taken into account for evaluating the conditional. Given that caching is allowed, that seems counterproductive.

I think what needs to be said is that just like elsewhere, conditionals here are evaluated against the representation generated by the QUERY.