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 $18.54   13.444%   $21.0325   $21.03   $21.05   $21.00  
  Purchase #2 $14.29   10.870%   $15.8433   $15.84   $15.85   $15.80  
  Purchase #3 $89.85   9.742%   $98.6032   $98.60   $98.60   $98.60  
  Purchase #4 $155.31   3.220%   $160.311   $160.31   $160.30   $160.30  
  Purchase #5 $158.39   6.244%   $168.2799   $168.28   $168.30   $168.30  
  Purchase #6 $31.87   9.232%   $34.8122   $34.81   $34.80   $34.80  
  Purchase #7 $158.25   3.203%   $163.3187   $163.32   $163.30   $163.30  
  7 purchase totals = $626.50       $662.2008   $662.19   $662.20   $662.10