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 $25.30   10.757%   $28.0215   $28.02   $28.00   $28.00  
  Purchase #2 $10.60   3.105%   $10.9291   $10.93   $10.95   $10.90  
  Purchase #3 $121.58   6.220%   $129.1423   $129.14   $129.15   $129.10  
  Purchase #4 $45.28   4.564%   $47.3466   $47.35   $47.35   $47.30  
  Purchase #5 $213.65   5.522%   $225.4478   $225.45   $225.45   $225.40  
  Purchase #6 $136.82   7.488%   $147.0651   $147.07   $147.05   $147.10  
  Purchase #7 $90.34   5.448%   $95.2617   $95.26   $95.25   $95.30  
  7 purchase totals = $643.57       $683.2141   $683.22   $683.20   $683.10