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 $242.33   11.406%   $269.9702   $269.97   $269.95   $270.00  
  Purchase #2 $135.95   12.200%   $152.5359   $152.54   $152.55   $152.50  
  Purchase #3 $161.61   5.452%   $170.421   $170.42   $170.40   $170.40  
  Purchase #4 $92.28   3.565%   $95.5698   $95.57   $95.55   $95.60  
  Purchase #5 $203.46   7.426%   $218.5689   $218.57   $218.55   $218.60  
  Purchase #6 $250.97   12.742%   $282.9486   $282.95   $282.95   $282.90  
  Purchase #7 $231.97   13.734%   $263.8288   $263.83   $263.85   $263.80  
  7 purchase totals = $1,318.57       $1,453.8432   $1,453.85   $1,453.80   $1,453.80