Both should be optimized. Usually it’s OD between 0.5 except if your protein is unstable, precipitates or has cleavage problems, in which case you can try a lower OD. IPTG depends on your protein and plasmid. For instance 0.2mM is recommended for pGEX, but I've seen many times that 0.05 is enough for a good quality sample for unstable proteins. For pQE 1mM is recommended, again I've seen that it works better with a lower concentration, 0.2mM in general. I always try 2 different colonies, 2 OD (for example 0.5 and 0.7), 3 IPTG concentrations (0.05, 0.2, 0.5), two temperatures (30 and 37°C), 2 times for induction (which depends on the stability of the protein, I’ve used between 30 min and 6 hours, lower when less stable). Each condition in 5ml tubes, comparing solubility and cleavage, after lysis and centrifugation by western blotting and SDS page.@Aditi konar