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 $78.94   13.771%   $89.8108   $89.81   $89.80   $89.80  
  Purchase #2 $181.41   10.810%   $201.0204   $201.02   $201.00   $201.00  
  Purchase #3 $42.92   10.682%   $47.5047   $47.50   $47.50   $47.50  
  Purchase #4 $53.26   5.468%   $56.1723   $56.17   $56.15   $56.20  
  Purchase #5 $72.61   9.472%   $79.4876   $79.49   $79.50   $79.50  
  Purchase #6 $95.30   6.596%   $101.586   $101.59   $101.60   $101.60  
  Purchase #7 $38.51   13.647%   $43.7655   $43.77   $43.75   $43.80  
  7 purchase totals = $562.95       $619.3473   $619.35   $619.30   $619.40