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 $114.86   12.581%   $129.3105   $129.31   $129.30   $129.30  
  Purchase #2 $213.96   6.332%   $227.5079   $227.51   $227.50   $227.50  
  Purchase #3 $67.63   6.334%   $71.9137   $71.91   $71.90   $71.90  
  Purchase #4 $240.32   12.781%   $271.0353   $271.04   $271.05   $271.00  
  Purchase #5 $175.59   4.322%   $183.179   $183.18   $183.20   $183.20  
  Purchase #6 $9.77   4.633%   $10.2226   $10.22   $10.20   $10.20  
  Purchase #7 $140.94   3.642%   $146.073   $146.07   $146.05   $146.10  
  7 purchase totals = $963.07       $1,039.2420   $1,039.24   $1,039.20   $1,039.20