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 $131.85   5.778%   $139.4683   $139.47   $139.45   $139.50  
  Purchase #2 $97.94   4.644%   $102.4883   $102.49   $102.50   $102.50  
  Purchase #3 $14.77   7.383%   $15.8605   $15.86   $15.85   $15.90  
  Purchase #4 $82.42   4.498%   $86.1273   $86.13   $86.15   $86.10  
  Purchase #5 $203.37   8.579%   $220.8171   $220.82   $220.80   $220.80  
  Purchase #6 $93.37   3.368%   $96.5147   $96.51   $96.50   $96.50  
  Purchase #7 $146.92   9.395%   $160.7231   $160.72   $160.70   $160.70  
  7 purchase totals = $770.64       $821.9993   $822.00   $821.95   $822.00