Top tube bags are both very useful and very Fredly on a racy road bike. A handlebar bag can be even more useful, has a lot more capacity and is significantly less Fredly unless you like to look like a racer.
Finding a good seat bag can be a bit tricky, so many of them are badly designed. I find the ones with two Velcro straps that pull the bag up tightly against the saddle often sway a lot less than ones with buckles. I like the Bontrager Pro and Serfas Slimline bags.