Anti Motoring protection of steam generator

Generally speaking, yes. Why would you want to keep the field energized since the turbine trip should have already taken the entire unit off-line.

I was wondering whether no load excitation would be a better option than no excitation. What if mechanical trips are of transient nature?

It's a matter of control/operational philosophy. Something caused the trip to occur, therefore most operators would want to know why. If you are getting nuisance trips and you immediately try to resynchronize without regard for the cause then be my guest. If your trip closes the steam and/or fuel supply valves then it will be some time before you will be ready to resynchronize, and resetting the excitation system will be the quickest item in your startup checklist.

But the number one reason to not keep the excitation on the field is overheating of the generator iron due to overexcitation (the Volts per Hertz problem) and reduced cooling as the speed drops. From ABB... http://www.abb.com/product/db0003db004281/c12573e700330419c1256e77002fb8f5.aspx "...Overvoltage, underfrequency, or a combination of the two can result in excessive flux density in the generator or transformer iron, producing heat that can reach damaging levels. Heating damage to the transformer or generator, if severe enough, can result in costly equipment outages..." It's your call.

Thanks for discussion and answers.