Hi Aspirant!
it is not necessary to lock all the choices of filled colleges in counselling registration.
if you leave them then after an given time period the system will be automaticaly locked your filled choices.
if you filled your college choices and after that don't want to add college and preference of the college then you can surely locked it.

eg. if you have to locked choices on 5 am and you fill your choices and forget to locked then your choices will be automatically locked by system at 5 am.

If you are still confusing to select the proper colleges please visit this link so you can clear your doubt regarding selection of colleges.

https://medicine.careers360.com/neet-college-predictor

Hope This Helps.
THANKS!