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 $7.84   6.758%   $8.3698   $8.37   $8.35   $8.40  
  Purchase #2 $5.96   13.866%   $6.7864   $6.79   $6.80   $6.80  
  Purchase #3 $36.37   3.636%   $37.6924   $37.69   $37.70   $37.70  
  Purchase #4 $42.78   8.481%   $46.4082   $46.41   $46.40   $46.40  
  Purchase #5 $107.45   11.831%   $120.1624   $120.16   $120.15   $120.20  
  Purchase #6 $32.81   7.857%   $35.3879   $35.39   $35.40   $35.40  
  Purchase #7 $129.81   11.395%   $144.6018   $144.60   $144.60   $144.60  
  7 purchase totals = $363.02       $399.4089   $399.41   $399.40   $399.50