Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $53.87   12.135%   $60.4071   $60.41   $60.40   $60.40  
  Purchase #2 $50.57   9.147%   $55.1956   $55.20   $55.20   $55.20  
  Purchase #3 $115.35   7.651%   $124.1754   $124.18   $124.20   $124.20  
  Purchase #4 $172.85   12.646%   $194.7086   $194.71   $194.70   $194.70  
  Purchase #5 $119.72   3.432%   $123.8288   $123.83   $123.85   $123.80  
  Purchase #6 $13.31   8.838%   $14.4863   $14.49   $14.50   $14.50  
  Purchase #7 $223.50   12.719%   $251.927   $251.93   $251.95   $251.90  
  7 purchase totals = $749.17       $824.7288   $824.75   $824.80   $824.70