“We recommend booking as early as possible,” Mr. Weinberg said, adding that users have saved an average of 25 percent in the past two years when the website was in development. “The longer time horizon we have to look for better deals, the better off we are.”

Hotel rates can be volatile, but catching a decline is often a matter of luck or diligence. Independently, Mr. Harteveldt recently checked on a five-day reservation in Atlanta and found the daily rate had dropped $50, so he rebooked. If HotelSlash “costs $30 a year and they find a less expensive rate for the same hotel and the same room or better and it’s more than the $30 membership, it pays for itself with one stay,” he said.

The upshot

As with most travel bookings, those travelers who work the hardest — meaning they do a lot of comparison searches and price checks — tend to save the most.

Many hotel booking sites aim to reduce the work by offering price guarantees. Booking.com will match a lower price found elsewhere, but the onus is on the traveler to find it and pursue a refund. For a fee, Hopper will allow you to freeze a price at a hotel and if it rises will pay the difference. Fees vary and are applied to any booking, but forfeited if a user does not book the room. In taking on price monitoring, HotelSlash handles the later-stage legwork, often for less.

But consumers should always do comparison searches in making the initial booking.

Exploring HotelSlash, I found some rates that were better than at hotel websites. I sometimes found similar rates at other online agencies. Other times, I found slightly better rates at hotel sites directly, but comparing them required scrutiny of the terms. For example, for a three-night refundable stay in June at the Paris Las Vegas Resort & Casino in Las Vegas, I got a quote of $831, including taxes, but excluding nearly $138 in resort fees for a total of $969 from HotelSlash; including taxes and fees, Expedia’s price was $981. An initial quote of $702 from the hotel itself grew to $952 with taxes and fees once I clicked through to the payment page.