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 $18.81   6.559%   $20.0437   $20.04   $20.05   $20.00  
  Purchase #2 $13.53   7.513%   $14.5465   $14.55   $14.55   $14.50  
  Purchase #3 $28.70   3.166%   $29.6086   $29.61   $29.60   $29.60  
  Purchase #4 $68.57   5.758%   $72.5183   $72.52   $72.50   $72.50  
  Purchase #5 $213.69   12.375%   $240.1341   $240.13   $240.15   $240.10  
  Purchase #6 $8.30   10.466%   $9.1687   $9.17   $9.15   $9.20  
  Purchase #7 $187.39   9.326%   $204.866   $204.87   $204.85   $204.90  
  7 purchase totals = $538.99       $590.8859   $590.89   $590.85   $590.80