Here is a table showing dielectric absorption for different capacitor materials. This more important if your sample and hold is not working right than for timing capacitors but it may be useful to some of you.
NP0 material is actually not that great for absorption. But it does have good temperature behavior.
For ceramics caps, use the highest voltage range you can accommodate on the board. Higher voltage parts will have less voltage coefficient effects than low voltage parts. Use 50V or 100V parts instead of 10V parts. They are also more robust.