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 $249.53   4.167%   $259.9279   $259.93   $259.95   $259.90  
  Purchase #2 $88.97   5.216%   $93.6107   $93.61   $93.60   $93.60  
  Purchase #3 $122.62   5.473%   $129.331   $129.33   $129.35   $129.30  
  Purchase #4 $162.51   8.645%   $176.559   $176.56   $176.55   $176.60  
  Purchase #5 $143.60   9.567%   $157.3382   $157.34   $157.35   $157.30  
  Purchase #6 $20.91   9.269%   $22.8481   $22.85   $22.85   $22.80  
  Purchase #7 $93.46   8.361%   $101.2742   $101.27   $101.25   $101.30  
  7 purchase totals = $881.60       $940.8891   $940.89   $940.90   $940.80