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 $228.61   13.420%   $259.2895   $259.29   $259.30   $259.30  
  Purchase #2 $79.99   10.835%   $88.6569   $88.66   $88.65   $88.70  
  Purchase #3 $90.76   13.346%   $102.8728   $102.87   $102.85   $102.90  
  Purchase #4 $180.63   7.200%   $193.6354   $193.64   $193.65   $193.60  
  Purchase #5 $241.50   8.435%   $261.8705   $261.87   $261.85   $261.90  
  Purchase #6 $145.49   4.872%   $152.5783   $152.58   $152.60   $152.60  
  Purchase #7 $226.75   3.344%   $234.3325   $234.33   $234.35   $234.30  
  7 purchase totals = $1,193.73       $1,293.2359   $1,293.24   $1,293.25   $1,293.30