Is it considered good or bad practice to raise the flaps right after touchdown?
You should wait until clearing the runway before performing any non-essential checklist items because you are still in a critical phase of flight and at a relatively high risk while still on the runway.  You should focus 100% of your attention to controlling the airplane (don't stop flying the airplane until you come to a complete stop) and watching for other traffic (or someone crossing the runway in front of you) that might be a hazard.
There is also the possibility that in a retractable gear airplane you could accidently retract the gear if you are moving things while still preoccupied with landing.  Once you exit the runway and come to a complete stop, you can devote the proper attention to the after landing checklist so will be more likely to complete it correctly without missing any items that may become an issue on your next flight.
That being said, follow the guidance in your POH.  Sometimes there are abnormal situations that actually call for this, like voretaq7 mentions in his answer, the Cherokee POH references it as a short field landing technique.
FAA Guidance
The FAA also covers this in AC 91-73B - Parts 91 and 135 Single Pilot, Flight
School Procedures During Taxi Operations.   It says (in part):

7. SINGLE-PILOT PROCEDURES.
a. General.
...
In addition, upon  landing, no items should be changed or moved until
the entire aircraft has crossed the respective  runway’s hold short
line.
...
NOTE: After the entire aircraft crosses over the landing runway’s hold  short line, conduct after-landing checklist items, based upon
company  procedures, before contacting ATC for taxi instructions.

You could also fail a checkride for doing this
The PTS also repeatedly stresses the  objective:

Utilizes after landing runway incursion avoidance procedures.

To figure out what they mean by that, we can look at their Runway Incursion Avoidance material, which says:

Finally, after landing and upon exiting  the runway, ensure your
aircraft has completely crossed over  the runway hold markings. Once
all parts of the aircraft have  crossed the runway holding position
markings, you must hold  unless further instructions have been issued
by ATC. Do not  initiate non-essential communications or actions until
the  aircraft has stopped and the brakes set.