It does not matter since in top-tier consulting you will always be on probation, even if the official period has ended.
You will be subject to constant performance feedback
- weekly or bi-weekly feedback on a project
- bi-annual 'official' reviews
and if your performance is not sufficient or you don't progress fast enough, you will be CTL'd (counseled to leave). There are few steps in-between, yet up-or-out is real to a certain extend.
That being said, I know only very few people that got actually asked to leave in the first few years at McK. Most leave in the first few months, the rest sticks around. Partially, because recruiting and interviewing evaluates who can do the job and who cannot.
In short, I would not worry about it.