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 $18.59   10.468%   $20.536   $20.54   $20.55   $20.50  
  Purchase #2 $142.51   3.378%   $147.324   $147.32   $147.30   $147.30  
  Purchase #3 $198.30   13.569%   $225.2073   $225.21   $225.20   $225.20  
  Purchase #4 $199.73   10.338%   $220.3781   $220.38   $220.40   $220.40  
  Purchase #5 $7.45   6.155%   $7.9085   $7.91   $7.90   $7.90  
  Purchase #6 $36.79   13.708%   $41.8332   $41.83   $41.85   $41.80  
  Purchase #7 $38.90   7.271%   $41.7284   $41.73   $41.75   $41.70  
  7 purchase totals = $642.27       $704.9155   $704.92   $704.95   $704.80