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 $112.32   10.582%   $124.2057   $124.21   $124.20   $124.20  
  Purchase #2 $185.39   8.650%   $201.4262   $201.43   $201.45   $201.40  
  Purchase #3 $27.53   6.589%   $29.344   $29.34   $29.35   $29.30  
  Purchase #4 $35.81   5.702%   $37.8519   $37.85   $37.85   $37.90  
  Purchase #5 $131.55   5.357%   $138.5971   $138.60   $138.60   $138.60  
  Purchase #6 $200.57   7.566%   $215.7451   $215.75   $215.75   $215.70  
  Purchase #7 $246.76   7.381%   $264.9734   $264.97   $264.95   $265.00  
  7 purchase totals = $939.93       $1,012.1434   $1,012.15   $1,012.15   $1,012.10