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 $121.75   8.221%   $131.7591   $131.76   $131.75   $131.80  
  Purchase #2 $115.34   3.102%   $118.9178   $118.92   $118.90   $118.90  
  Purchase #3 $81.95   13.472%   $92.9903   $92.99   $93.00   $93.00  
  Purchase #4 $169.66   10.591%   $187.6287   $187.63   $187.65   $187.60  
  Purchase #5 $204.69   4.162%   $213.2092   $213.21   $213.20   $213.20  
  Purchase #6 $19.59   8.558%   $21.2665   $21.27   $21.25   $21.30  
  Purchase #7 $222.70   5.176%   $234.227   $234.23   $234.25   $234.20  
  7 purchase totals = $935.68       $999.9986   $1,000.01   $1,000.00   $1,000.00