When to make reservations for Cinqueterre?

If you find someplace you like the earlier the better.
In the 5 villages combined there are limited choices, the best choices will fill up fast ; how fast partially depends on the place as some will probably take a May 2018 booking today and others will tell you they do have the rates or calendar yet for May and as a result not show up on some of the booking engines as available.
There are no chains or large hotels, all of the places are small. Outside of Monterosso there are not even really hotels in the other towns more like B&B's mostly without breakfast.
Monterosso you do have more options than the other towns so not saying if you wait you will miss out but I don't see sales or pricing fluctuations so if you find a place you like I would not wait.

If the place allows you to modify or cancel than all the better ; even if you don't think you will it is nice to have the freedom to tweak your plans as you get closer to your dates.

Kathy,

I'd suggest starting to make inquiries in October or November of this year, before the various hotels and so on close for the season. If you wait too long, you may not get a reply for a long time.

May is fairly early in the season, so that should provide a better opportunity to find a vacancy. Monterosso would be an excellent choice, as it's the largest of the five towns and therefore has the greatest number of hotels and other tourist amenities (and the best beaches).

As the previous reply mentioned, Monterosso is the only town with conventional hotels (desk staff, elevators), as accommodations in the other towns are mostly just small rooms and some don't provide breakfast. I like a good breakfast before going out touring, so that's a deal-breaker for me.

If you have the RS Italy guidebook, there's a good range of accommodations listed there for all the towns. The next edition of the Italy guidebook will be released in November 2017.

Seconding all of the above advice: I'd put the giddiyup on and book this fall. Seems I remember some January posts from worried OP's who were finding many of the choice accommodations already booked up for May. We came very close to having that issue as well.

I'll also second Ken's preference for Monterosso for the reasons he has listed. We enjoyed La Spiaggia very much for its excellent location (very easy walk from the station), proximity to good restaurants (Cantina di Miky is a fave!) and lovely desk staff (Maria is a gem, and Mr. Andrea is a veritable font of local knowledge). Rooms are small and not fancy (very clean, tidy and comfortable, though) but book one with a sea-view balcony and you won't spend any time in it anyway! I'd recommended it to two couples we know for their CT trips and they both loved it as well. Good value, breakfast included, and it has a lift!!!

Start looking at what you might want so when you do firm up your dates, you can go right to what you want. Or at least have a 1st, 2nd , 3rd choice lined up, so you aren't starting from scratch.

Sooner will be better than later.

Thanks to all for your informative replies! I think I will get a move on and reserve something by fall.