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 $245.64   13.853%   $279.6685   $279.67   $279.65   $279.70  
  Purchase #2 $108.73   12.163%   $121.9548   $121.95   $121.95   $122.00  
  Purchase #3 $165.38   6.870%   $176.7416   $176.74   $176.75   $176.70  
  Purchase #4 $66.76   11.654%   $74.5402   $74.54   $74.55   $74.50  
  Purchase #5 $221.87   7.141%   $237.7137   $237.71   $237.70   $237.70  
  Purchase #6 $140.56   13.616%   $159.6986   $159.70   $159.70   $159.70  
  Purchase #7 $167.36   3.152%   $172.6352   $172.64   $172.65   $172.60  
  7 purchase totals = $1,116.30       $1,222.9526   $1,222.95   $1,222.95   $1,222.90