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 $19.99   9.870%   $21.963   $21.96   $21.95   $22.00  
  Purchase #2 $225.65   4.645%   $236.1314   $236.13   $236.15   $236.10  
  Purchase #3 $132.80   6.535%   $141.4785   $141.48   $141.50   $141.50  
  Purchase #4 $72.97   7.417%   $78.3822   $78.38   $78.40   $78.40  
  Purchase #5 $28.55   10.727%   $31.6126   $31.61   $31.60   $31.60  
  Purchase #6 $117.97   11.673%   $131.7406   $131.74   $131.75   $131.70  
  Purchase #7 $133.28   7.284%   $142.9881   $142.99   $143.00   $143.00  
  7 purchase totals = $731.21       $784.2964   $784.29   $784.35   $784.30