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 $56.78   11.727%   $63.4386   $63.44   $63.45   $63.40  
  Purchase #2 $35.73   5.596%   $37.7295   $37.73   $37.75   $37.70  
  Purchase #3 $206.55   13.183%   $233.7795   $233.78   $233.80   $233.80  
  Purchase #4 $136.43   13.109%   $154.3146   $154.31   $154.30   $154.30  
  Purchase #5 $184.99   8.742%   $201.1618   $201.16   $201.15   $201.20  
  Purchase #6 $35.97   10.328%   $39.685   $39.69   $39.70   $39.70  
  Purchase #7 $127.43   9.831%   $139.9576   $139.96   $139.95   $140.00  
  7 purchase totals = $783.88       $870.0666   $870.07   $870.10   $870.10