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 $51.71   13.307%   $58.591   $58.59   $58.60   $58.60  
  Purchase #2 $138.93   10.556%   $153.5955   $153.60   $153.60   $153.60  
  Purchase #3 $101.48   7.215%   $108.8018   $108.80   $108.80   $108.80  
  Purchase #4 $140.83   7.781%   $151.788   $151.79   $151.80   $151.80  
  Purchase #5 $24.82   5.734%   $26.2432   $26.24   $26.25   $26.20  
  Purchase #6 $69.70   3.501%   $72.1402   $72.14   $72.15   $72.10  
  Purchase #7 $61.64   3.519%   $63.8091   $63.81   $63.80   $63.80  
  7 purchase totals = $589.11       $634.9688   $634.97   $635.00   $634.90