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 $34.78   13.124%   $39.3445   $39.34   $39.35   $39.30  
  Purchase #2 $206.93   9.514%   $226.6173   $226.62   $226.60   $226.60  
  Purchase #3 $77.28   8.683%   $83.9902   $83.99   $84.00   $84.00  
  Purchase #4 $124.68   8.627%   $135.4361   $135.44   $135.45   $135.40  
  Purchase #5 $201.48   3.107%   $207.74   $207.74   $207.75   $207.70  
  Purchase #6 $59.42   12.586%   $66.8986   $66.90   $66.90   $66.90  
  Purchase #7 $115.50   8.182%   $124.9502   $124.95   $124.95   $125.00  
  7 purchase totals = $820.07       $884.9769   $884.98   $885.00   $884.90