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 $135.96   8.717%   $147.8116   $147.81   $147.80   $147.80  
  Purchase #2 $104.49   6.713%   $111.5044   $111.50   $111.50   $111.50  
  Purchase #3 $129.76   10.311%   $143.1396   $143.14   $143.15   $143.10  
  Purchase #4 $174.28   7.867%   $187.9906   $187.99   $188.00   $188.00  
  Purchase #5 $127.63   9.221%   $139.3988   $139.40   $139.40   $139.40  
  Purchase #6 $232.83   5.730%   $246.1712   $246.17   $246.15   $246.20  
  Purchase #7 $12.47   6.455%   $13.2749   $13.27   $13.25   $13.30  
  7 purchase totals = $917.42       $989.2911   $989.28   $989.25   $989.30