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 $33.98   12.391%   $38.1905   $38.19   $38.20   $38.20  
  Purchase #2 $77.37   3.251%   $79.8853   $79.89   $79.90   $79.90  
  Purchase #3 $53.66   5.637%   $56.6848   $56.68   $56.70   $56.70  
  Purchase #4 $226.30   5.813%   $239.4548   $239.45   $239.45   $239.50  
  Purchase #5 $29.34   3.797%   $30.454   $30.45   $30.45   $30.50  
  Purchase #6 $172.71   10.226%   $190.3713   $190.37   $190.35   $190.40  
  Purchase #7 $220.37   4.566%   $230.4321   $230.43   $230.45   $230.40  
  7 purchase totals = $813.73       $865.4728   $865.46   $865.50   $865.60