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 $241.71   8.197%   $261.523   $261.52   $261.50   $261.50  
  Purchase #2 $188.69   7.734%   $203.2833   $203.28   $203.30   $203.30  
  Purchase #3 $16.65   11.454%   $18.5571   $18.56   $18.55   $18.60  
  Purchase #4 $184.38   10.291%   $203.3545   $203.35   $203.35   $203.40  
  Purchase #5 $225.30   9.607%   $246.9446   $246.94   $246.95   $246.90  
  Purchase #6 $146.31   5.561%   $154.4463   $154.45   $154.45   $154.40  
  Purchase #7 $243.71   3.158%   $251.4064   $251.41   $251.40   $251.40  
  7 purchase totals = $1,246.75       $1,339.5152   $1,339.51   $1,339.50   $1,339.50