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 $223.91   11.571%   $249.8186   $249.82   $249.80   $249.80  
  Purchase #2 $232.93   7.307%   $249.9502   $249.95   $249.95   $250.00  
  Purchase #3 $32.73   9.505%   $35.841   $35.84   $35.85   $35.80  
  Purchase #4 $175.90   4.300%   $183.4637   $183.46   $183.45   $183.50  
  Purchase #5 $148.69   13.401%   $168.6159   $168.62   $168.60   $168.60  
  Purchase #6 $120.45   10.600%   $133.2177   $133.22   $133.20   $133.20  
  Purchase #7 $227.39   12.364%   $255.5045   $255.50   $255.50   $255.50  
  7 purchase totals = $1,162.00       $1,276.4116   $1,276.41   $1,276.35   $1,276.40