You don't need a reservoir, but they may help reduce temps. Depending on how many components you're cooling will depend on whether or not it'd be a good idea. If you're just doing your CPU, I wouldn't bother. If you're doing anything more, I'd just purchase a small reservoir.
If you want your system to hold more fluid, just make the tubing a bit longer then it needs to be. That should work just fine. T-Connectors in the configuration you speak of are generally used as fill ports, but will hold some fluid as well.