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 $6.60   8.149%   $7.1378   $7.14   $7.15   $7.10  
  Purchase #2 $107.60   13.329%   $121.942   $121.94   $121.95   $121.90  
  Purchase #3 $235.61   9.435%   $257.8398   $257.84   $257.85   $257.80  
  Purchase #4 $208.64   3.320%   $215.5668   $215.57   $215.55   $215.60  
  Purchase #5 $7.60   10.110%   $8.3684   $8.37   $8.35   $8.40  
  Purchase #6 $11.92   12.719%   $13.4361   $13.44   $13.45   $13.40  
  Purchase #7 $240.59   10.156%   $265.0243   $265.02   $265.00   $265.00  
  7 purchase totals = $818.56       $889.3152   $889.32   $889.30   $889.20