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 $18.87   9.395%   $20.6428   $20.64   $20.65   $20.60  
  Purchase #2 $94.76   8.563%   $102.8743   $102.87   $102.85   $102.90  
  Purchase #3 $6.29   5.557%   $6.6395   $6.64   $6.65   $6.60  
  Purchase #4 $30.70   7.277%   $32.934   $32.93   $32.95   $32.90  
  Purchase #5 $75.95   5.307%   $79.9807   $79.98   $80.00   $80.00  
  Purchase #6 $224.34   10.820%   $248.6136   $248.61   $248.60   $248.60  
  Purchase #7 $130.94   13.324%   $148.3864   $148.39   $148.40   $148.40  
  7 purchase totals = $581.85       $640.0713   $640.06   $640.10   $640.00