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 $18.90   13.581%   $21.4668   $21.47   $21.45   $21.50  
  Purchase #2 $132.70   13.774%   $150.9781   $150.98   $151.00   $151.00  
  Purchase #3 $47.60   3.116%   $49.0832   $49.08   $49.10   $49.10  
  Purchase #4 $150.29   3.159%   $155.0377   $155.04   $155.05   $155.00  
  Purchase #5 $206.89   3.735%   $214.6173   $214.62   $214.60   $214.60  
  Purchase #6 $140.74   6.636%   $150.0795   $150.08   $150.10   $150.10  
  Purchase #7 $141.90   7.397%   $152.3963   $152.40   $152.40   $152.40  
  7 purchase totals = $839.02       $893.6589   $893.67   $893.70   $893.70