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 $181.54   4.356%   $189.4479   $189.45   $189.45   $189.40  
  Purchase #2 $7.88   13.669%   $8.9571   $8.96   $8.95   $9.00  
  Purchase #3 $71.36   13.584%   $81.0535   $81.05   $81.05   $81.10  
  Purchase #4 $131.56   5.354%   $138.6037   $138.60   $138.60   $138.60  
  Purchase #5 $16.89   11.287%   $18.7964   $18.80   $18.80   $18.80  
  Purchase #6 $239.57   6.631%   $255.4559   $255.46   $255.45   $255.50  
  Purchase #7 $186.90   4.337%   $195.0059   $195.01   $195.00   $195.00  
  7 purchase totals = $835.70       $887.3204   $887.33   $887.30   $887.40