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 $236.40   13.777%   $268.9688   $268.97   $268.95   $269.00  
  Purchase #2 $179.76   7.196%   $192.6955   $192.70   $192.70   $192.70  
  Purchase #3 $24.25   11.322%   $26.9956   $27.00   $27.00   $27.00  
  Purchase #4 $213.69   3.495%   $221.1585   $221.16   $221.15   $221.20  
  Purchase #5 $115.61   3.615%   $119.7893   $119.79   $119.80   $119.80  
  Purchase #6 $225.28   12.660%   $253.8004   $253.80   $253.80   $253.80  
  Purchase #7 $39.41   11.280%   $43.8554   $43.86   $43.85   $43.90  
  7 purchase totals = $1,034.40       $1,127.2635   $1,127.28   $1,127.25   $1,127.40