Over and Under Biasing as means for tone control

Well in consumer-land, tone control refers to bass and treble balance controls. I'm not sure bias will affect  this in that sense.

Under-biasing risks excessive distortion, over bias risks excessive heat and shortened (possibly VERY short) tube life.

Harmonics control may I say?
if you are talking about power tubes I highly discourage overbiasing to warm up
the sound.  It just gives you supernova and runaway tubes easily. 
Gain and driver tubes are a different story.  You can get pretty good returns from say biasing a 6SN7 from say 2mA to 4mA depending on B+ and plate resistor.  However since a lot of modern designs are direct coupled.  Increasing bias at gain stage by tweaking grid voltage or cathode resistor usually means decreasing bias at driver stage.  Not for the faint of heart.  

I read somewhere that under biasing tbe Joule 100 takes a bit of sparkle off, which might be desiarable with some speakers or source players. 

I use this to make sure bias is exactly what mfg. recommends

http://www.vhtamp.com/products/vht-accessories/tube-tester-2

Worth every penny, helps also rule out a bad tube or cathode resistor.  Not a tube tester but very useful .