branch merging tactics (lock repository or not?)

[David Everly]

We currently have 3 active development branches as well as our trunk.
Our trunk holds our next major release.  Branches A, B, and C hold our
next three major releases (in that order).  About once a week, we merge
the incremental changes (since the last merge) from the trunk, to branch
A, then from branch A to branch B, then from branch B to branch C.  We
also have bug fix branches for releases currently in production, and if
one of them has a new release on it, we merge (or consider merging)
these changes into the trunk.

I have been locking the repository while we do these merges, but some
developers are complaining.

Is it a good or bad practice to lock while merging branches.  If we do
not lock the repository while merging branches, what would be the
potential result?