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 $18.68   11.646%   $20.8555   $20.86   $20.85   $20.90  
  Purchase #2 $90.72   9.547%   $99.381   $99.38   $99.40   $99.40  
  Purchase #3 $71.32   12.321%   $80.1073   $80.11   $80.10   $80.10  
  Purchase #4 $7.56   11.647%   $8.4405   $8.44   $8.45   $8.40  
  Purchase #5 $176.92   7.290%   $189.8175   $189.82   $189.80   $189.80  
  Purchase #6 $156.49   7.811%   $168.7134   $168.71   $168.70   $168.70  
  Purchase #7 $122.41   11.500%   $136.4872   $136.49   $136.50   $136.50  
  7 purchase totals = $644.10       $703.8024   $703.81   $703.80   $703.80