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 $56.32   5.842%   $59.6102   $59.61   $59.60   $59.60  
  Purchase #2 $89.97   3.564%   $93.1765   $93.18   $93.20   $93.20  
  Purchase #3 $239.31   13.438%   $271.4685   $271.47   $271.45   $271.50  
  Purchase #4 $49.25   7.565%   $52.9758   $52.98   $53.00   $53.00  
  Purchase #5 $22.89   3.537%   $23.6996   $23.70   $23.70   $23.70  
  Purchase #6 $71.75   6.758%   $76.5989   $76.60   $76.60   $76.60  
  Purchase #7 $23.39   8.674%   $25.4188   $25.42   $25.40   $25.40  
  7 purchase totals = $552.88       $602.9483   $602.96   $602.95   $603.00