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 $174.42   8.868%   $189.8876   $189.89   $189.90   $189.90  
  Purchase #2 $152.49   4.159%   $158.8321   $158.83   $158.85   $158.80  
  Purchase #3 $109.88   5.451%   $115.8696   $115.87   $115.85   $115.90  
  Purchase #4 $210.87   8.476%   $228.7433   $228.74   $228.75   $228.70  
  Purchase #5 $171.32   13.554%   $194.5407   $194.54   $194.55   $194.50  
  Purchase #6 $135.62   7.773%   $146.1617   $146.16   $146.15   $146.20  
  Purchase #7 $96.63   10.789%   $107.0554   $107.06   $107.05   $107.10  
  7 purchase totals = $1,051.23       $1,141.0904   $1,141.09   $1,141.10   $1,141.10