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 $18.49   12.305%   $20.7652   $20.77   $20.75   $20.80  
  Purchase #2 $80.52   5.291%   $84.7803   $84.78   $84.80   $84.80  
  Purchase #3 $232.46   12.195%   $260.8085   $260.81   $260.80   $260.80  
  Purchase #4 $30.35   12.718%   $34.2099   $34.21   $34.20   $34.20  
  Purchase #5 $39.97   13.216%   $45.2524   $45.25   $45.25   $45.30  
  Purchase #6 $160.42   6.246%   $170.4398   $170.44   $170.45   $170.40  
  Purchase #7 $104.43   12.576%   $117.5631   $117.56   $117.55   $117.60  
  7 purchase totals = $666.64       $733.8192   $733.82   $733.80   $733.90