Leader lenght changes for me for each species and every type of situation.
For Flatheads, I prefer a shorter leader,6 to 9″.
A couple reasons for that is one, its so much easier to cast. You bait is usually large and the sinker heavy. When pitched through the air, the centrifugal force usually ends up tossing that bait a bit further than your hook.
Tangles also seem to be a problem with heavier sinkers and baits twirling through the air.
I have also taken a liking to the “Dark 30 rig”. No leader at all. That really makes casting much easier.
Even though I prefer shorter leaders on flathead rigs, I still pay out a bit of line to let the bait swim freely depending on the area that I am fishing. If its snaggy I will keep a tight leash on my little pet, but if the bottom is fairly clean, i’ll let him run around a play a bit.
On a long leash, if the tip stops thumping indicating that the bait has become less active a few pulls on the line will get him going again, or I’ll shorten up the leash.
Dave makes a great point on hook sets with long leaders. If you don’t make sure you have a tight line or direct contact with the fish before setting the hook, your going to wiff. I’ll take up all the slack and wait untill I am sure that there is no bow in the line, before setting the hook.
This is also a good argument for using the lightest sinker you can get away with. Its the sinker changing the angle of the line that prevents a good hookset.
If the fish start dropping the bait before I get a tight line, then I’ll think about shortening up the leash. However, I beleive dropped baits are typically smaller fish, channels and turtles. Hey, you have to blame somebody!
Channel cats are a different story. I typically use a longer leader, anywhere from 18 to 36″ depending on what I am fishing.
Snags get the shorter end and current edges and chutes the longer. WIth dead bait it will tend to end up on the bottom and not leave a good scent trail. Getting the bait up and waving around in the current is a more natural presentation and I belive the cats respond to it better. After all when cats are feeding in current they are looking for food to come downstream waving along in the current.
There is one time of year that I will use a long leader on snags and even “feed” line into them. THats the spawn. When those cats are holed up tight and deep in the snags, I have found that getting right in their face will often trigger a bite. Its often a very light bite, but if you paying attention you can detect it and hook up.