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 $49.64   11.401%   $55.2995   $55.30   $55.30   $55.30  
  Purchase #2 $125.95   9.256%   $137.6079   $137.61   $137.60   $137.60  
  Purchase #3 $110.66   7.635%   $119.1089   $119.11   $119.10   $119.10  
  Purchase #4 $248.73   4.229%   $259.2488   $259.25   $259.25   $259.20  
  Purchase #5 $112.45   7.297%   $120.6555   $120.66   $120.65   $120.70  
  Purchase #6 $126.43   12.429%   $142.144   $142.14   $142.15   $142.10  
  Purchase #7 $135.79   5.698%   $143.5273   $143.53   $143.55   $143.50  
  7 purchase totals = $909.65       $977.5919   $977.60   $977.60   $977.50