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 $82.79   9.401%   $90.5731   $90.57   $90.55   $90.60  
  Purchase #2 $108.99   5.545%   $115.0335   $115.03   $115.05   $115.00  
  Purchase #3 $138.92   9.160%   $151.6451   $151.65   $151.65   $151.60  
  Purchase #4 $109.65   11.720%   $122.501   $122.50   $122.50   $122.50  
  Purchase #5 $15.26   4.178%   $15.8976   $15.90   $15.90   $15.90  
  Purchase #6 $173.74   6.631%   $185.2607   $185.26   $185.25   $185.30  
  Purchase #7 $5.25   12.728%   $5.9182   $5.92   $5.90   $5.90  
  7 purchase totals = $634.60       $686.8292   $686.83   $686.80   $686.80