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 $228.68   13.480%   $259.5061   $259.51   $259.50   $259.50  
  Purchase #2 $40.29   8.539%   $43.7304   $43.73   $43.75   $43.70  
  Purchase #3 $14.35   9.467%   $15.7085   $15.71   $15.70   $15.70  
  Purchase #4 $84.56   9.537%   $92.6245   $92.62   $92.60   $92.60  
  Purchase #5 $128.53   5.799%   $135.9835   $135.98   $136.00   $136.00  
  Purchase #6 $247.25   13.487%   $280.5966   $280.60   $280.60   $280.60  
  Purchase #7 $151.28   4.558%   $158.1753   $158.18   $158.20   $158.20  
  7 purchase totals = $894.94       $986.3249   $986.33   $986.35   $986.30