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 $27.63   5.127%   $29.0466   $29.05   $29.05   $29.00  
  Purchase #2 $19.41   12.768%   $21.8883   $21.89   $21.90   $21.90  
  Purchase #3 $196.67   9.566%   $215.4835   $215.48   $215.50   $215.50  
  Purchase #4 $55.45   4.670%   $58.0395   $58.04   $58.05   $58.00  
  Purchase #5 $187.93   4.616%   $196.6048   $196.60   $196.60   $196.60  
  Purchase #6 $162.56   6.292%   $172.7883   $172.79   $172.80   $172.80  
  Purchase #7 $146.62   6.258%   $155.7955   $155.80   $155.80   $155.80  
  7 purchase totals = $796.27       $849.6465   $849.65   $849.70   $849.60