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 $19.98   10.471%   $22.0721   $22.07   $22.05   $22.10  
  Purchase #2 $228.32   4.291%   $238.1172   $238.12   $238.10   $238.10  
  Purchase #3 $18.99   7.501%   $20.4144   $20.41   $20.40   $20.40  
  Purchase #4 $214.25   10.703%   $237.1812   $237.18   $237.20   $237.20  
  Purchase #5 $61.99   12.474%   $69.7226   $69.72   $69.70   $69.70  
  Purchase #6 $20.57   13.324%   $23.3107   $23.31   $23.30   $23.30  
  Purchase #7 $28.29   13.292%   $32.0503   $32.05   $32.05   $32.10  
  7 purchase totals = $592.39       $642.8685   $642.86   $642.80   $642.90