Hosts are just like other people. Their answer to your question will be just as myriad as the world's population! Here's the general rule, however. If a host is Instant Book feel free to instant book, but it is nice to explain about your trip and the people coming with you in your first message. Instant book hosts still like to know about their guests so they can plan accordingly. For instance, honeymooners might get flowers, vacationers might get brochures, etc. Also, an Instant Book host still has the right to refuse accomodations if they feel uncomfortable with the guest so you do want to get off on the right foot.
Any host who is not instant book will require an inquiry.
Here are some ideas to include in your first message to any host:
1. The reason for your trip to my town.
2. Who is in your group (wife, husband, brothers, sisters, best friends, etc.).
3. Approximate arrival time.
4. Any questions you may have about my place.
5. Any questions about the local area such as things to see or visit or where to eat. Be sure to lists some of your interests, though so we can tailor our reply to what would be best for you.
6. Any special needs you may have. (People with allergies have requested we remove offending foods.)
And when all is said and done- be ready for a great trip staying in a great host's home!