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 $236.67   13.390%   $268.3601   $268.36   $268.35   $268.40  
  Purchase #2 $117.96   3.234%   $121.7748   $121.77   $121.75   $121.80  
  Purchase #3 $53.54   5.236%   $56.3434   $56.34   $56.35   $56.30  
  Purchase #4 $29.56   3.487%   $30.5908   $30.59   $30.60   $30.60  
  Purchase #5 $8.63   8.527%   $9.3659   $9.37   $9.35   $9.40  
  Purchase #6 $246.42   9.194%   $269.0759   $269.08   $269.10   $269.10  
  Purchase #7 $30.59   3.500%   $31.6607   $31.66   $31.65   $31.70  
  7 purchase totals = $723.37       $787.1716   $787.17   $787.15   $787.30