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 $27.43   9.360%   $29.9974   $30.00   $30.00   $30.00  
  Purchase #2 $16.69   9.688%   $18.3069   $18.31   $18.30   $18.30  
  Purchase #3 $27.47   12.218%   $30.8263   $30.83   $30.85   $30.80  
  Purchase #4 $28.81   7.208%   $30.8866   $30.89   $30.90   $30.90  
  Purchase #5 $213.87   9.871%   $234.9811   $234.98   $235.00   $235.00  
  Purchase #6 $15.45   4.773%   $16.1874   $16.19   $16.20   $16.20  
  Purchase #7 $203.32   4.293%   $212.0485   $212.05   $212.05   $212.00  
  7 purchase totals = $533.04       $573.2342   $573.25   $573.30   $573.20