A tapered custom watch strap is a strap that is one width at the lugs (where the strap attaches to the watch) and tapers down to a narrower width at the buckle such as 24/22, meaning 24mm wide at the lugs and 22mm wide at the buckle. Most of the standard Panerai straps are tapered to 22mm, even on the larger 47mm watches. A straight strap is one that's the same width at the lugs and the buckle such as 24/24 meaning 24mm wide at the lugs and 24mm wide at the buckle. The big question is... why and when would you want either of these styles of custom watch straps?
It's (mostly) about what type of watch your strap is going on
The fastest way to decide if you want a tapered or straight custom watch band is to think about what kind of watch your strap is going on. For larger watches such as Panerai and Bell and Ross a straight strap is usually more appropriate as it will better suit the general design of the watch as well as help stand up to and enhance the robust, large style of the watch. For smaller watches such as Rolex and IWC, a tapered strap may be more appropriate so that the strap doesn't overwhelm the watch and look out of place. However, this isn't a strict rule. Often people are looking for a handmade custom watch strap that will help their smaller watch look larger and more robust, in which case a straight strap may be the right choice.
Sometimes it's about the setting your watch strap will be in
Often, though, where you're going to be wearing your watch with the custom strap is a large factor too. In an office environment, where a suit and tie is standard and a huge hunk of wide leather might look awkward a tapered strap may be the right choice, particularly for a larger watch in order to help tone down the overall size of the package presented on the wrist. Alternately, if you're mostly a jeans-and-T-shirt person a tapered strap might look a bit wonky on a casual watch that's never going to see the inside of an office building. Watches are, after all, an accessory and should be matched as well as possible with the surroundings and attire.
Personal style trumps all else
The main thing to keep in mind when choosing the width of your custom leather watch strap is that your own personal style is the ultimate arbiter of what will work best for you. Some people, regardless of wrist size or whether they're wearing a suit or not, will be able to pull off a 47mm Panerai with a straight strap all day every day. Others will lean towards a more refined look at all times, and will be happier with a tapered watch band that better suits their quieter style. There is no right or wrong here, there's only what you're willing to try!