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 $55.97   6.763%   $59.7553   $59.76   $59.75   $59.80  
  Purchase #2 $122.72   10.763%   $135.9284   $135.93   $135.95   $135.90  
  Purchase #3 $11.89   6.626%   $12.6778   $12.68   $12.70   $12.70  
  Purchase #4 $202.72   8.329%   $219.6045   $219.60   $219.60   $219.60  
  Purchase #5 $135.81   4.852%   $142.3995   $142.40   $142.40   $142.40  
  Purchase #6 $126.95   12.455%   $142.7616   $142.76   $142.75   $142.80  
  Purchase #7 $115.41   7.664%   $124.255   $124.26   $124.25   $124.30  
  7 purchase totals = $771.47       $837.3821   $837.39   $837.40   $837.50