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 $240.85   6.430%   $256.3367   $256.34   $256.35   $256.30  
  Purchase #2 $135.86   4.751%   $142.3147   $142.31   $142.30   $142.30  
  Purchase #3 $118.99   5.506%   $125.5416   $125.54   $125.55   $125.50  
  Purchase #4 $224.46   4.672%   $234.9468   $234.95   $234.95   $234.90  
  Purchase #5 $57.33   12.108%   $64.2715   $64.27   $64.25   $64.30  
  Purchase #6 $132.86   12.559%   $149.5459   $149.55   $149.55   $149.50  
  Purchase #7 $78.40   10.240%   $86.4282   $86.43   $86.45   $86.40  
  7 purchase totals = $988.75       $1,059.3854   $1,059.39   $1,059.40   $1,059.20