I find it ludicrous that people can't just give you an answer. It should be noted that linearity is very important but you didn't ask for that information.
The OP asked for guidelines in regard to frequency response. Addressing linearity is FAR more important that upper/lower extension, and is equally relevant. So that is answering his question. As a guideline, deviation from flat is the spec that should be focused upon.
Please consider that the frequency response should be accommodated by the room in which they will be placed in.
True, room interaction is important ... but there's no way to determine that from a published spec.