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 $48.77   12.759%   $54.9926   $54.99   $55.00   $55.00  
  Purchase #2 $173.49   9.793%   $190.4799   $190.48   $190.50   $190.50  
  Purchase #3 $127.53   8.412%   $138.2578   $138.26   $138.25   $138.30  
  Purchase #4 $143.38   6.353%   $152.4889   $152.49   $152.50   $152.50  
  Purchase #5 $244.54   10.413%   $270.004   $270.00   $270.00   $270.00  
  Purchase #6 $196.62   9.553%   $215.4031   $215.40   $215.40   $215.40  
  Purchase #7 $51.26   9.650%   $56.2066   $56.21   $56.20   $56.20  
  7 purchase totals = $985.59       $1,077.8329   $1,077.83   $1,077.85   $1,077.90