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 $218.90   9.543%   $239.7896   $239.79   $239.80   $239.80  
  Purchase #2 $191.48   13.177%   $216.7113   $216.71   $216.70   $216.70  
  Purchase #3 $65.82   5.807%   $69.6422   $69.64   $69.65   $69.60  
  Purchase #4 $236.35   9.806%   $259.5265   $259.53   $259.55   $259.50  
  Purchase #5 $52.54   13.278%   $59.5163   $59.52   $59.50   $59.50  
  Purchase #6 $105.27   13.572%   $119.5572   $119.56   $119.55   $119.60  
  Purchase #7 $34.95   4.649%   $36.5748   $36.57   $36.55   $36.60  
  7 purchase totals = $905.31       $1,001.3179   $1,001.32   $1,001.30   $1,001.30