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 $213.69   12.741%   $240.9162   $240.92   $240.90   $240.90  
  Purchase #2 $32.89   5.163%   $34.5881   $34.59   $34.60   $34.60  
  Purchase #3 $192.25   5.854%   $203.5043   $203.50   $203.50   $203.50  
  Purchase #4 $71.44   4.509%   $74.6612   $74.66   $74.65   $74.70  
  Purchase #5 $156.83   9.833%   $172.2511   $172.25   $172.25   $172.30  
  Purchase #6 $243.64   7.413%   $261.701   $261.70   $261.70   $261.70  
  Purchase #7 $12.29   7.747%   $13.2421   $13.24   $13.25   $13.20  
  7 purchase totals = $923.03       $1,000.8640   $1,000.86   $1,000.85   $1,000.90