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 $125.78   6.862%   $134.411   $134.41   $134.40   $134.40  
  Purchase #2 $21.50   11.631%   $24.0007   $24.00   $24.00   $24.00  
  Purchase #3 $234.84   8.308%   $254.3505   $254.35   $254.35   $254.40  
  Purchase #4 $117.58   13.379%   $133.311   $133.31   $133.30   $133.30  
  Purchase #5 $133.43   7.696%   $143.6988   $143.70   $143.70   $143.70  
  Purchase #6 $30.77   6.619%   $32.8067   $32.81   $32.80   $32.80  
  Purchase #7 $236.95   5.489%   $249.9562   $249.96   $249.95   $250.00  
  7 purchase totals = $900.85       $972.5349   $972.54   $972.50   $972.60