Hello Dipankar,
                       For critical matching ----->
1.  they should be placed in the same diffusion region,
2.  with a interdigited or a common-centroid layout for better matching...
3.  Well shielded with dummies on their sides, to protect them from  
       WPE/STI effects [This matching of WPE should be done on Post-
           Layout  Netlist ]

--Mayank