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 $21.42   11.818%   $23.9514   $23.95   $23.95   $24.00  
  Purchase #2 $66.98   10.129%   $73.7644   $73.76   $73.75   $73.80  
  Purchase #3 $237.42   12.320%   $266.6701   $266.67   $266.65   $266.70  
  Purchase #4 $158.74   4.798%   $166.3563   $166.36   $166.35   $166.40  
  Purchase #5 $239.73   3.759%   $248.7415   $248.74   $248.75   $248.70  
  Purchase #6 $33.27   5.713%   $35.1707   $35.17   $35.15   $35.20  
  Purchase #7 $18.65   5.185%   $19.617   $19.62   $19.60   $19.60  
  7 purchase totals = $776.21       $834.2714   $834.27   $834.20   $834.40