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 $23.63   12.322%   $26.5417   $26.54   $26.55   $26.50  
  Purchase #2 $136.32   13.588%   $154.8432   $154.84   $154.85   $154.80  
  Purchase #3 $52.98   7.535%   $56.972   $56.97   $56.95   $57.00  
  Purchase #4 $231.95   8.291%   $251.181   $251.18   $251.20   $251.20  
  Purchase #5 $152.36   7.216%   $163.3543   $163.35   $163.35   $163.40  
  Purchase #6 $44.58   8.328%   $48.2926   $48.29   $48.30   $48.30  
  Purchase #7 $135.50   5.706%   $143.2316   $143.23   $143.25   $143.20  
  7 purchase totals = $777.32       $844.4164   $844.40   $844.45   $844.40