I have never used IB, all my guests send requests. My situation is a little different from yours in that my listing is a private room in my home for 1 guest only. So I don't have the worries entire-home hosts have as far as guests being able to sneak in extra people, pets, throw parties, etc.
When a guest sends a request, the message I send them varies depending on what sort of initial message the guest has sent along with their request, as well as what I see on their profile page. If a guest sends a nice, informative message, i.e. "Hi Sarah, I found your listing and it sounds like exactly what I'm looking for. I really need to escape the plummeting temps where I live and look forward to some beach time and just relaxing with a book in a quiet spot. The 20 minute walk to town isn't a issue for me- I love to walk and am the outdoorsy type. I look forward to meeting you and your dog."
A message like that tells me that this is an articulate, communicative person who understands that a home-share host would want to know something about a guest who is asking to stay with them, and mentioning the walk to town and the dog makes it evident that they've read through my listing description, as those things aren't mentioned in the part of the listing info they see without clicking on "Read more". If they also have a page or two full of reviews from hosts stating what a lovely guest this was, I wouldn't have any questions of the guest at that point, I would just accept their request and message them back thanking them for their request, that I look forward to meeting them, and that I'll send them a map to my place and transportation info when it gets closer to their check-in date.
If their first message only says something like "I'll be arriving around noon", when I haven't even accepted their request yet, then my response would be quite different- I would solicit more info. " Hi XX, thanks for your request. I'm just checking that you are aware that I have a dog and a cat (in case of allergies), that the room is for one person only and that.... ". What I'm doing there is trying to determine if they have actually read through the listing info thoroughly. That message usually results in a much more informative reply.
If I see they are new to the platform and/or have no reviews, my first message to them may be more in the way of making sure they understand what staying at an Airbnb means. "Hi XX, Thanks for your request. I see you are new to the platform, so I just want to ensure that you understand how it all works- have you read all through my listing info and house rules? Checked the terms of my cancellation policy? " Etc.
You can create saved messages where you re-iterate all your house rules (this would be more important for entire place hosts- I have almost no "rules") and whatever else you want to convey to guests so you don't have to write everything out each time- the you can just add to it depending on the individual request and any questions they may have asked.
Because you have to either accept or decline a booking request within 24 hours, if a guest you'd like more info from before making a decision doesn't respond soon, you could message again letting them now you require an answer by XX time or you'll have to decline.
Also be aware that if you receive a Inquiry, as opposed to Request, there's no need to either pre-approve or decline- just answering an inquiry message within 24 hours fulfills your obligations.
Declining does lower your Acceptance rate, but don't worry too much about that- that stat isn't included in the Superhost metrics, and while hosts do get warning messages from Airbnb for declining too many requests, it isn't something that gets hosts delisted unless you are declining tons. Always try to get guests to withdraw requests that are inappropriate (they are asking to bring a pet when you don't allow them, kids in a no-kids listing, more guests than you allow, etc) so you don't have to waste a decline on them. "Hi XX, No, sorry, I can't waive my no-pets rule. I get a lot of guests who are allergic to pet dander, as is my cleaner. Please withdraw this request ASAP so you'll be free to look for a pet-friendly listing- I know there are quite a few in my area".
@Michelle1648