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 $246.49   8.724%   $267.9938   $267.99   $268.00   $268.00  
  Purchase #2 $44.29   5.855%   $46.8832   $46.88   $46.90   $46.90  
  Purchase #3 $165.77   12.501%   $186.4929   $186.49   $186.50   $186.50  
  Purchase #4 $77.64   10.759%   $85.9933   $85.99   $86.00   $86.00  
  Purchase #5 $36.73   5.306%   $38.6789   $38.68   $38.70   $38.70  
  Purchase #6 $54.33   6.568%   $57.8984   $57.90   $57.90   $57.90  
  Purchase #7 $221.72   7.156%   $237.5863   $237.59   $237.60   $237.60  
  7 purchase totals = $846.97       $921.5268   $921.52   $921.60   $921.60