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 $234.57   6.250%   $249.2306   $249.23   $249.25   $249.20  
  Purchase #2 $192.80   4.724%   $201.9079   $201.91   $201.90   $201.90  
  Purchase #3 $58.91   12.791%   $66.4452   $66.45   $66.45   $66.40  
  Purchase #4 $80.69   4.386%   $84.2291   $84.23   $84.25   $84.20  
  Purchase #5 $7.83   7.704%   $8.4332   $8.43   $8.45   $8.40  
  Purchase #6 $16.90   8.268%   $18.2973   $18.30   $18.30   $18.30  
  Purchase #7 $196.72   6.439%   $209.3868   $209.39   $209.40   $209.40  
  7 purchase totals = $788.42       $837.9301   $837.94   $838.00   $837.80