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 $7.80   5.195%   $8.2052   $8.21   $8.20   $8.20  
  Purchase #2 $112.61   10.105%   $123.9892   $123.99   $124.00   $124.00  
  Purchase #3 $103.49   6.337%   $110.0482   $110.05   $110.05   $110.00  
  Purchase #4 $129.85   8.257%   $140.5717   $140.57   $140.55   $140.60  
  Purchase #5 $76.59   6.307%   $81.4205   $81.42   $81.40   $81.40  
  Purchase #6 $229.82   12.252%   $257.9775   $257.98   $258.00   $258.00  
  Purchase #7 $82.47   11.492%   $91.9475   $91.95   $91.95   $91.90  
  7 purchase totals = $742.63       $814.1598   $814.17   $814.15   $814.10