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 $84.97   8.636%   $92.308   $92.31   $92.30   $92.30  
  Purchase #2 $11.86   6.578%   $12.6402   $12.64   $12.65   $12.60  
  Purchase #3 $197.86   11.851%   $221.3084   $221.31   $221.30   $221.30  
  Purchase #4 $23.65   10.684%   $26.1768   $26.18   $26.20   $26.20  
  Purchase #5 $45.89   5.407%   $48.3713   $48.37   $48.35   $48.40  
  Purchase #6 $192.75   11.680%   $215.2632   $215.26   $215.25   $215.30  
  Purchase #7 $122.78   9.297%   $134.1949   $134.19   $134.20   $134.20  
  7 purchase totals = $679.76       $750.2628   $750.26   $750.25   $750.30