If your team does not require a merge and you are the first developer owning a parallel version to check in a parallel, then you might or might not need to merge later. You can merge checked-in versions only. If there is one other parallel version and it is in the working state, then the other developer must merge changes with yours when completing the task. If the parallel version is checked in, then you can either merge now or later. If there are two or more versions, then you might merge a parallel version later. The merge depends on the state of the other versions.
If your team does not require a merge and you are not the last developer owning a parallel version to check in a parallel, merge later.
One way to remember that you have a parallel version to merge is to start the History dialog box for the version to merge. You can watch your version and the state of the other parallels.