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 $242.42   13.745%   $275.7406   $275.74   $275.75   $275.70  
  Purchase #2 $140.86   11.619%   $157.2265   $157.23   $157.25   $157.20  
  Purchase #3 $236.33   9.556%   $258.9137   $258.91   $258.90   $258.90  
  Purchase #4 $32.52   9.370%   $35.5671   $35.57   $35.55   $35.60  
  Purchase #5 $234.99   6.510%   $250.2878   $250.29   $250.30   $250.30  
  Purchase #6 $236.92   9.485%   $259.3919   $259.39   $259.40   $259.40  
  Purchase #7 $26.77   12.598%   $30.1425   $30.14   $30.15   $30.10  
  7 purchase totals = $1,150.81       $1,267.2701   $1,267.27   $1,267.30   $1,267.20