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 $171.83   3.162%   $177.2633   $177.26   $177.25   $177.30  
  Purchase #2 $131.96   6.601%   $140.6707   $140.67   $140.65   $140.70  
  Purchase #3 $22.30   9.216%   $24.3552   $24.36   $24.35   $24.40  
  Purchase #4 $208.56   11.197%   $231.9125   $231.91   $231.90   $231.90  
  Purchase #5 $242.47   8.677%   $263.5091   $263.51   $263.50   $263.50  
  Purchase #6 $220.76   3.496%   $228.4778   $228.48   $228.50   $228.50  
  Purchase #7 $239.89   6.638%   $255.8139   $255.81   $255.80   $255.80  
  7 purchase totals = $1,237.77       $1,322.0025   $1,322.00   $1,321.95   $1,322.10