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 $98.92   5.536%   $104.3962   $104.40   $104.40   $104.40  
  Purchase #2 $128.56   12.688%   $144.8717   $144.87   $144.85   $144.90  
  Purchase #3 $167.81   9.507%   $183.7637   $183.76   $183.75   $183.80  
  Purchase #4 $135.33   3.797%   $140.4685   $140.47   $140.45   $140.50  
  Purchase #5 $35.87   9.545%   $39.2938   $39.29   $39.30   $39.30  
  Purchase #6 $144.49   11.118%   $160.5544   $160.55   $160.55   $160.60  
  Purchase #7 $207.90   4.233%   $216.7004   $216.70   $216.70   $216.70  
  7 purchase totals = $918.88       $990.0487   $990.04   $990.00   $990.20