But development for Leopard is also nicer because you can now use Objective-C 2.0 and garbage collection and lots of other new classes, like e.g. NSOperation which helps development for multi-core processing.
..if you're just relying on Core Animation for a few animations, you can write those yourself. And if you're not going to use Obj-C 2.0 GC for performance reasons, strike that off.
You can easily put Leopard-specific features/hacks in, too. I developed ShoveBox for Tiger, but I've had to add in some logic around how the app works with Spaces.
...but if, by the time you're done, the target audience of your app will have already gotten Leopard, it might be useful to just target it. There are a lot of useful enhancements all around the API that will save you time.
But the developers tell me that Leopard offers a lot of animations and stuff (CoreSomethingSomething? haha) that Tiger does not do.