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 $200.78   10.286%   $221.4322   $221.43   $221.45   $221.40  
  Purchase #2 $130.69   11.319%   $145.4828   $145.48   $145.50   $145.50  
  Purchase #3 $217.60   3.518%   $225.2552   $225.26   $225.25   $225.30  
  Purchase #4 $79.63   13.528%   $90.4023   $90.40   $90.40   $90.40  
  Purchase #5 $149.76   10.222%   $165.0685   $165.07   $165.05   $165.10  
  Purchase #6 $84.65   12.706%   $95.4056   $95.41   $95.40   $95.40  
  Purchase #7 $40.92   3.554%   $42.3743   $42.37   $42.35   $42.40  
  7 purchase totals = $904.03       $985.4209   $985.42   $985.40   $985.50