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 $71.93   12.224%   $80.7227   $80.72   $80.70   $80.70  
  Purchase #2 $38.79   7.125%   $41.5538   $41.55   $41.55   $41.60  
  Purchase #3 $33.47   3.174%   $34.5323   $34.53   $34.55   $34.50  
  Purchase #4 $178.42   12.756%   $201.1793   $201.18   $201.20   $201.20  
  Purchase #5 $7.93   3.844%   $8.2348   $8.23   $8.25   $8.20  
  Purchase #6 $106.64   8.840%   $116.067   $116.07   $116.05   $116.10  
  Purchase #7 $103.97   3.509%   $107.6183   $107.62   $107.60   $107.60  
  7 purchase totals = $541.15       $589.9082   $589.90   $589.90   $589.90