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 $46.91   4.437%   $48.9914   $48.99   $49.00   $49.00  
  Purchase #2 $28.99   8.341%   $31.4081   $31.41   $31.40   $31.40  
  Purchase #3 $16.28   4.645%   $17.0362   $17.04   $17.05   $17.00  
  Purchase #4 $90.72   12.307%   $101.8849   $101.88   $101.90   $101.90  
  Purchase #5 $37.33   4.819%   $39.1289   $39.13   $39.15   $39.10  
  Purchase #6 $154.68   12.289%   $173.6886   $173.69   $173.70   $173.70  
  Purchase #7 $88.39   12.873%   $99.7684   $99.77   $99.75   $99.80  
  7 purchase totals = $463.30       $511.9065   $511.91   $511.95   $511.90