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 $244.89   12.871%   $276.4098   $276.41   $276.40   $276.40  
  Purchase #2 $175.37   10.242%   $193.3314   $193.33   $193.35   $193.30  
  Purchase #3 $164.87   3.520%   $170.6734   $170.67   $170.65   $170.70  
  Purchase #4 $182.49   11.616%   $203.688   $203.69   $203.70   $203.70  
  Purchase #5 $167.82   12.160%   $188.2269   $188.23   $188.25   $188.20  
  Purchase #6 $70.70   5.641%   $74.6882   $74.69   $74.70   $74.70  
  Purchase #7 $172.69   11.492%   $192.5355   $192.54   $192.55   $192.50  
  7 purchase totals = $1,178.83       $1,299.5532   $1,299.56   $1,299.60   $1,299.50