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 $202.98   10.659%   $224.6156   $224.62   $224.60   $224.60  
  Purchase #2 $120.49   5.313%   $126.8916   $126.89   $126.90   $126.90  
  Purchase #3 $208.95   5.839%   $221.1506   $221.15   $221.15   $221.20  
  Purchase #4 $76.74   6.114%   $81.4319   $81.43   $81.45   $81.40  
  Purchase #5 $140.72   3.419%   $145.5312   $145.53   $145.55   $145.50  
  Purchase #6 $107.72   5.726%   $113.888   $113.89   $113.90   $113.90  
  Purchase #7 $49.80   13.406%   $56.4762   $56.48   $56.50   $56.50  
  7 purchase totals = $907.40       $969.9851   $969.99   $970.05   $970.00