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 $239.51   12.439%   $269.3026   $269.30   $269.30   $269.30  
  Purchase #2 $96.58   6.621%   $102.9746   $102.97   $102.95   $103.00  
  Purchase #3 $15.53   7.154%   $16.641   $16.64   $16.65   $16.60  
  Purchase #4 $237.86   4.600%   $248.8016   $248.80   $248.80   $248.80  
  Purchase #5 $131.91   8.469%   $143.0815   $143.08   $143.10   $143.10  
  Purchase #6 $47.69   6.619%   $50.8466   $50.85   $50.85   $50.80  
  Purchase #7 $9.55   12.611%   $10.7544   $10.75   $10.75   $10.80  
  7 purchase totals = $778.63       $842.4023   $842.39   $842.40   $842.40