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 $10.50   13.552%   $11.923   $11.92   $11.90   $11.90  
  Purchase #2 $73.78   5.321%   $77.7058   $77.71   $77.70   $77.70  
  Purchase #3 $209.86   3.574%   $217.3604   $217.36   $217.35   $217.40  
  Purchase #4 $9.74   9.705%   $10.6853   $10.69   $10.70   $10.70  
  Purchase #5 $109.48   10.318%   $120.7761   $120.78   $120.80   $120.80  
  Purchase #6 $19.80   6.728%   $21.1321   $21.13   $21.15   $21.10  
  Purchase #7 $95.72   3.211%   $98.7936   $98.79   $98.80   $98.80  
  7 purchase totals = $528.88       $558.3763   $558.38   $558.40   $558.40