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 $17.55   4.730%   $18.3801   $18.38   $18.40   $18.40  
  Purchase #2 $139.47   6.497%   $148.5314   $148.53   $148.55   $148.50  
  Purchase #3 $139.42   6.301%   $148.2049   $148.20   $148.20   $148.20  
  Purchase #4 $131.28   9.180%   $143.3315   $143.33   $143.35   $143.30  
  Purchase #5 $68.93   9.820%   $75.6989   $75.70   $75.70   $75.70  
  Purchase #6 $74.45   5.580%   $78.6043   $78.60   $78.60   $78.60  
  Purchase #7 $31.71   13.277%   $35.9201   $35.92   $35.90   $35.90  
  7 purchase totals = $602.81       $648.6712   $648.66   $648.70   $648.60