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 $247.77   13.421%   $281.0232   $281.02   $281.00   $281.00  
  Purchase #2 $119.91   11.627%   $133.8519   $133.85   $133.85   $133.90  
  Purchase #3 $24.25   8.854%   $26.3971   $26.40   $26.40   $26.40  
  Purchase #4 $83.80   3.728%   $86.9241   $86.92   $86.90   $86.90  
  Purchase #5 $53.57   7.601%   $57.6419   $57.64   $57.65   $57.60  
  Purchase #6 $178.84   4.377%   $186.6678   $186.67   $186.65   $186.70  
  Purchase #7 $44.87   12.847%   $50.6344   $50.63   $50.65   $50.60  
  7 purchase totals = $753.01       $823.1404   $823.13   $823.10   $823.10