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 $245.48   13.766%   $279.2728   $279.27   $279.25   $279.30  
  Purchase #2 $213.56   11.256%   $237.5983   $237.60   $237.60   $237.60  
  Purchase #3 $112.95   8.613%   $122.6784   $122.68   $122.70   $122.70  
  Purchase #4 $231.64   9.153%   $252.842   $252.84   $252.85   $252.80  
  Purchase #5 $26.41   13.359%   $29.9381   $29.94   $29.95   $29.90  
  Purchase #6 $166.78   4.284%   $173.9249   $173.92   $173.90   $173.90  
  Purchase #7 $172.78   4.142%   $179.9365   $179.94   $179.95   $179.90  
  7 purchase totals = $1,169.60       $1,276.1910   $1,276.19   $1,276.20   $1,276.10