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 $242.89   12.417%   $273.0497   $273.05   $273.05   $273.00  
  Purchase #2 $176.86   10.738%   $195.8512   $195.85   $195.85   $195.90  
  Purchase #3 $236.42   7.523%   $254.2059   $254.21   $254.20   $254.20  
  Purchase #4 $43.26   6.315%   $45.9919   $45.99   $46.00   $46.00  
  Purchase #5 $80.57   12.426%   $90.5816   $90.58   $90.60   $90.60  
  Purchase #6 $77.71   8.611%   $84.4016   $84.40   $84.40   $84.40  
  Purchase #7 $52.46   7.290%   $56.2843   $56.28   $56.30   $56.30  
  7 purchase totals = $910.17       $1,000.3662   $1,000.36   $1,000.40   $1,000.40