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 $109.57   12.676%   $123.4591   $123.46   $123.45   $123.50  
  Purchase #2 $6.45   12.778%   $7.2742   $7.27   $7.25   $7.30  
  Purchase #3 $78.41   3.400%   $81.0759   $81.08   $81.10   $81.10  
  Purchase #4 $16.86   13.307%   $19.1036   $19.10   $19.10   $19.10  
  Purchase #5 $177.43   8.303%   $192.162   $192.16   $192.15   $192.20  
  Purchase #6 $27.28   11.763%   $30.4889   $30.49   $30.50   $30.50  
  Purchase #7 $122.72   5.484%   $129.45   $129.45   $129.45   $129.50  
  7 purchase totals = $538.72       $583.0137   $583.01   $583.00   $583.20