Do you always need a separate ground for the power part and the signal part
however heard that it is good practice to keep these two grounds separate

It is beneficial to have a star grounding scheme, read here: http://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html  this means (partially) separated grounds which all come together at one point (like battery ground for example).

The reason to do this is to separate the current loops to stop them interfering with other circuits.

In your case of a boost converter (also DCDC converters in general) you would want to separate the current loop at the output (switches, inductor, diode etc), from the control circuits like the uC.

If you don't separate these current loops, the current flowing in the "power" loop at the output can interfere with the uC. For example if the returning current of the ground connection of the output shares the ground connection of the uC. Then the ground of the uC might be "lifted" in voltage and/or have a lot of disturbance and noise on it. If the uC gets its own connection (star ground !) to a "quiet" grounding point then these problems would be eliminated.