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 $88.83   10.246%   $97.9315   $97.93   $97.95   $97.90  
  Purchase #2 $56.79   3.143%   $58.5749   $58.57   $58.55   $58.60  
  Purchase #3 $6.69   5.502%   $7.0581   $7.06   $7.05   $7.10  
  Purchase #4 $144.29   8.481%   $156.5272   $156.53   $156.55   $156.50  
  Purchase #5 $121.72   13.238%   $137.8333   $137.83   $137.85   $137.80  
  Purchase #6 $162.94   3.133%   $168.0449   $168.04   $168.05   $168.00  
  Purchase #7 $233.99   12.792%   $263.922   $263.92   $263.90   $263.90  
  7 purchase totals = $815.25       $889.8919   $889.88   $889.90   $889.80