Its a good practice use dc-dc converters and linear regulators? Or its better don't mix them?
Better use all linear tecnology or dc-dc converters in "cascade" or dc-dc with multiple outputs?
The short answer: Yes, it is fairly common to mix different types of regulators in the same design.
The long answer: It totally depends on what you are trying to do. Switching regulators are going to be more efficient, but linear regulators are going to be less noisy. If you are working with noise critical applications like audio or video processing, you might have to use linear regulators just to keep the signal from being so noisy.
Cascading regulators can help - using a switching regulator to drop most of the voltage, then a linear to create the desired level - but even this might produce too much oscillation on the power line from the switching frequency. This can often be controlled with a ferrite bead or other passive filter, but sometimes, straight linear is the only way to go.
On top of that, switching regulators are typically more expensive and use more components than linear ones, so when efficiency isn't an issue the component cost factor may be the deciding factor. However, as noted by AndrejaKo, efficiency does have a big impact on operational cost, and additional cooling consideration may need to be taken when using linear regulation due to the excessive heat dissipation which will also increase cost.