should i add a capacitor next to VCC at the IC's?

i saw in many circuits that there is an additional capacitor connected between vcc and GND, most capacitors are 10uF and someone told me they are used to filter unwanted noises and to keep vcc as clean as possible,

why using 10uF and not 0.1uF for example? i must use those capacitors in all of my circuits?

They are called decoupling (or bypass) capacitors and are used to suppress noise. The choice of the size will depend on the application and the power supply. 0.1 uF is usually a good size to start with, but if your circuit is still misbehaving, you can put a larger one in parallel.

As for their importance, one of our more prolific members here (sorry, I forget names), says something along the lines of "When an otherwise good looking circuit misbehaves, 9 out of 10 times the problem is poor decoupling. The remaining 1/10 will eventually turn out to be decoupling as well."

Author, sorry for butchering the quote.

No problem well remembered. http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Hehe, I was expecting you to show up and post the link to the full text. It's a good read.

Until recently 10uF caps did not work well at high frequencies. Now that you can get 10 uF chip caps, they would probably work fine for decoupling. However, you don't want too much capacitance because it can cause a huge current surge when the device starts up.

The too much capacitance is in the region of 100,000 uF not the piddling amounts we are talking about here.

It really depends on the regulator. I am currently working on a circuit with a relatively low current regulator that is slowed way down by 20 uF of capacitance. (Which is not an over current problem, but...)