Carbu Heat should be used when the OAT is between +20 and -5ºC with visible moisture present. Visible moisture would be in the clouds, rain or bad visibilty. If you have 20 Km visibility there would not be a lot of moisture in the air.
If you use it to prevent ice forming AND you have a Carburetor temperature indicator you should apply just enough heat to get the temperature 5 degrees C above the yellow arc.
If you use it to get rid of icing that has accumulated allready you should use full heat, then when icing has disipated adjust heat to 5º above the yellow arc.
If you have no carbu temp indicator I would use full heat.
You should not use carbu heat on the ground for a prolonged time unless absolutely necessary. Because as you may or may not know, when using carbu heat the air going to the carburator does no longer pass through the air filter and therefore you can suck dust, sand or other particles in the engine.
If you use carbu heat (certainly full and with high powersetting) it may be necessary to lean because otherwise the mixture becomes too rich.
However it all depends on the engine type. In my experience with the SF260 (Lycoming O-540-E4 A5) it is very prone to get carbu icing. A symptom of carbu icing in that aircraft is that when you change power from about 75% power to full power the engine would start running rough and hesitating while changing the throttle.
Using full heat in this aircraft results in a huge loss of power and a very rough running engine so leaning is then absolutely necessary.
On the other hand, the C152 is in my opinion less likely to build up carbu ice and if you use full carbu heat you do not notice a lot of diference in how the engine runs.
If have no experience in the PA28
Hope this helps.
Bart