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 $16.85   6.708%   $17.9803   $17.98   $18.00   $18.00  
  Purchase #2 $99.57   13.524%   $113.0358   $113.04   $113.05   $113.00  
  Purchase #3 $125.25   10.735%   $138.6956   $138.70   $138.70   $138.70  
  Purchase #4 $122.61   6.104%   $130.0941   $130.09   $130.10   $130.10  
  Purchase #5 $73.76   8.871%   $80.3032   $80.30   $80.30   $80.30  
  Purchase #6 $181.84   5.870%   $192.514   $192.51   $192.50   $192.50  
  Purchase #7 $147.84   13.798%   $168.239   $168.24   $168.25   $168.20  
  7 purchase totals = $767.72       $840.8620   $840.86   $840.90   $840.80