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 $117.88   8.600%   $128.0177   $128.02   $128.00   $128.00  
  Purchase #2 $130.41   5.717%   $137.8655   $137.87   $137.85   $137.90  
  Purchase #3 $16.74   13.655%   $19.0258   $19.03   $19.05   $19.00  
  Purchase #4 $130.83   6.839%   $139.7775   $139.78   $139.80   $139.80  
  Purchase #5 $6.83   5.397%   $7.1986   $7.20   $7.20   $7.20  
  Purchase #6 $58.25   10.539%   $64.389   $64.39   $64.40   $64.40  
  Purchase #7 $183.69   5.146%   $193.1427   $193.14   $193.15   $193.10  
  7 purchase totals = $644.63       $689.4168   $689.43   $689.45   $689.40