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 $227.87   11.596%   $254.2938   $254.29   $254.30   $254.30  
  Purchase #2 $74.43   12.423%   $83.6764   $83.68   $83.70   $83.70  
  Purchase #3 $113.65   5.143%   $119.495   $119.50   $119.50   $119.50  
  Purchase #4 $244.60   4.127%   $254.6946   $254.69   $254.70   $254.70  
  Purchase #5 $247.54   9.562%   $271.2098   $271.21   $271.20   $271.20  
  Purchase #6 $21.87   7.622%   $23.5369   $23.54   $23.55   $23.50  
  Purchase #7 $79.82   5.813%   $84.4599   $84.46   $84.45   $84.50  
  7 purchase totals = $1,009.78       $1,091.3664   $1,091.37   $1,091.40   $1,091.40