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 $235.43   12.233%   $264.2302   $264.23   $264.25   $264.20  
  Purchase #2 $95.78   7.834%   $103.2834   $103.28   $103.30   $103.30  
  Purchase #3 $116.27   8.568%   $126.232   $126.23   $126.25   $126.20  
  Purchase #4 $170.30   10.252%   $187.7592   $187.76   $187.75   $187.80  
  Purchase #5 $91.25   7.256%   $97.8711   $97.87   $97.85   $97.90  
  Purchase #6 $24.57   10.857%   $27.2376   $27.24   $27.25   $27.20  
  Purchase #7 $182.55   5.484%   $192.561   $192.56   $192.55   $192.60  
  7 purchase totals = $916.15       $999.1745   $999.17   $999.20   $999.20