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 $224.47   8.396%   $243.3165   $243.32   $243.30   $243.30  
  Purchase #2 $196.60   10.351%   $216.9501   $216.95   $216.95   $217.00  
  Purchase #3 $228.46   13.610%   $259.5534   $259.55   $259.55   $259.60  
  Purchase #4 $200.91   10.861%   $222.7308   $222.73   $222.75   $222.70  
  Purchase #5 $103.25   11.711%   $115.3416   $115.34   $115.35   $115.30  
  Purchase #6 $76.50   7.817%   $82.48   $82.48   $82.50   $82.50  
  Purchase #7 $120.45   7.827%   $129.8776   $129.88   $129.90   $129.90  
  7 purchase totals = $1,150.64       $1,270.2500   $1,270.25   $1,270.30   $1,270.30