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 $134.87   3.281%   $139.2951   $139.30   $139.30   $139.30  
  Purchase #2 $59.70   12.785%   $67.3326   $67.33   $67.35   $67.30  
  Purchase #3 $55.66   7.776%   $59.9881   $59.99   $60.00   $60.00  
  Purchase #4 $90.78   4.389%   $94.7643   $94.76   $94.75   $94.80  
  Purchase #5 $158.35   3.274%   $163.5344   $163.53   $163.55   $163.50  
  Purchase #6 $110.25   9.727%   $120.974   $120.97   $120.95   $121.00  
  Purchase #7 $114.26   9.644%   $125.2792   $125.28   $125.30   $125.30  
  7 purchase totals = $723.87       $771.1677   $771.16   $771.20   $771.20