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 $12.64   3.158%   $13.0392   $13.04   $13.05   $13.00  
  Purchase #2 $182.73   13.230%   $206.9052   $206.91   $206.90   $206.90  
  Purchase #3 $92.40   7.676%   $99.4926   $99.49   $99.50   $99.50  
  Purchase #4 $124.34   12.410%   $139.7706   $139.77   $139.75   $139.80  
  Purchase #5 $161.90   13.155%   $183.1979   $183.20   $183.20   $183.20  
  Purchase #6 $163.81   13.108%   $185.2822   $185.28   $185.30   $185.30  
  Purchase #7 $45.29   8.307%   $49.0522   $49.05   $49.05   $49.10  
  7 purchase totals = $783.11       $876.7399   $876.74   $876.75   $876.80