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 $47.63   9.621%   $52.2125   $52.21   $52.20   $52.20  
  Purchase #2 $136.39   9.492%   $149.3361   $149.34   $149.35   $149.30  
  Purchase #3 $193.54   4.551%   $202.348   $202.35   $202.35   $202.30  
  Purchase #4 $177.49   5.220%   $186.755   $186.76   $186.75   $186.80  
  Purchase #5 $117.71   8.109%   $127.2551   $127.26   $127.25   $127.30  
  Purchase #6 $36.56   7.633%   $39.3506   $39.35   $39.35   $39.40  
  Purchase #7 $141.38   9.830%   $155.2777   $155.28   $155.30   $155.30  
  7 purchase totals = $850.70       $912.5350   $912.55   $912.55   $912.60