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 $18.75   11.318%   $20.8721   $20.87   $20.85   $20.90  
  Purchase #2 $194.32   4.728%   $203.5074   $203.51   $203.50   $203.50  
  Purchase #3 $147.51   7.653%   $158.7989   $158.80   $158.80   $158.80  
  Purchase #4 $158.96   12.198%   $178.3499   $178.35   $178.35   $178.30  
  Purchase #5 $40.49   8.433%   $43.9045   $43.90   $43.90   $43.90  
  Purchase #6 $121.80   6.648%   $129.8973   $129.90   $129.90   $129.90  
  Purchase #7 $144.40   13.197%   $163.4565   $163.46   $163.45   $163.50  
  7 purchase totals = $826.23       $898.7866   $898.79   $898.75   $898.80