When rigging for the flats, it’s more a question of knowing how to deal with the current situation than anything else. For many flats fly fishers, the prepackaged tapered leaders made by many manufacturers are their go-to, and that often works just fine. They're hard to beat for convenience.
Personally I like to make my own leaders, mostly because I think I can do it just as well for less cost. Like most guys that roll their own leaders, I like to start with a heavy butt section. This allows for efficient transfer of energy from the flyline to the leader. I generally go with a 30-20-12 progression in pound test if I am expecting to be casting to tailing or cruising fish (reds, drum, trout) on the flat.
The typical leader I will use on the flats is around 9-10', but I have gone up to 14-15' in super clear water. Longer leaders/tippets become necessary on highly spooky/pressured fish, or when hunting the big sow speckled trout that will prowl the shallows from time to time. Windy conditions call for shorter, stouter leaders; something like an 8-9' leader of 50-30-20 or 40-20. When facing windy conditions, the heavier tippet turns over bulky, air-resistant flies a little better and can withstand the occasional windknot. This same configuration is what I will use when I am searching for fish with topwaters or sliders - the more casts you make, the more likely that you will eventually put a windknot in the line.The last thing you want to do is come tight on a good red after a crashing eat and have that tippet pop - ping! Don' ask me how I know this.
When I'm actually walking the flats, I will generally carry a small roll of 12-14lb tippet material, in case I need to refresh my tippet or I have some sort of catastrophic failure happen to my leader. Being unprepared for that when you're several hundred yards from the boat or truck is not something that you want to do to yourself. Don't ask me how I know. Just trust me when I say:
Fortune favors the prepared.