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 $234.94   10.794%   $260.2994   $260.30   $260.30   $260.30  
  Purchase #2 $62.97   6.296%   $66.9346   $66.93   $66.95   $66.90  
  Purchase #3 $49.95   11.750%   $55.8191   $55.82   $55.80   $55.80  
  Purchase #4 $72.55   13.245%   $82.1592   $82.16   $82.15   $82.20  
  Purchase #5 $193.49   12.796%   $218.249   $218.25   $218.25   $218.20  
  Purchase #6 $66.50   10.562%   $73.5237   $73.52   $73.50   $73.50  
  Purchase #7 $76.93   7.652%   $82.8167   $82.82   $82.80   $82.80  
  7 purchase totals = $757.33       $839.8017   $839.80   $839.75   $839.70