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 $186.69   9.710%   $204.8176   $204.82   $204.80   $204.80  
  Purchase #2 $90.55   3.146%   $93.3987   $93.40   $93.40   $93.40  
  Purchase #3 $60.94   11.867%   $68.1717   $68.17   $68.15   $68.20  
  Purchase #4 $159.78   6.149%   $169.6049   $169.60   $169.60   $169.60  
  Purchase #5 $59.67   11.632%   $66.6108   $66.61   $66.60   $66.60  
  Purchase #6 $185.92   6.638%   $198.2614   $198.26   $198.25   $198.30  
  Purchase #7 $228.61   4.461%   $238.8083   $238.81   $238.80   $238.80  
  7 purchase totals = $972.16       $1,039.6734   $1,039.67   $1,039.60   $1,039.70