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 $172.55   9.231%   $188.4781   $188.48   $188.50   $188.50  
  Purchase #2 $132.43   9.313%   $144.7632   $144.76   $144.75   $144.80  
  Purchase #3 $114.38   7.226%   $122.6451   $122.65   $122.65   $122.60  
  Purchase #4 $122.99   10.836%   $136.3172   $136.32   $136.30   $136.30  
  Purchase #5 $65.97   4.731%   $69.091   $69.09   $69.10   $69.10  
  Purchase #6 $36.84   13.797%   $41.9228   $41.92   $41.90   $41.90  
  Purchase #7 $5.45   13.280%   $6.1738   $6.17   $6.15   $6.20  
  7 purchase totals = $650.61       $709.3912   $709.39   $709.35   $709.40