Generally, it doesn't matter because the _time field that is created for the events in the summary index is always normalized to GMT epoch. There is one thing to consider, though; if you are using any snap-to definitions, these may be effected by the user's Time zone value. For example, if your TZ offset is not an even-hour (I just read that North Korea is dropping 30 minutes from their TZ), then @h for one user may be 30 minutes different than @h for another user. The same concern exists for relative day specifiers anywhere they exist (not just for the snap-to part) because where midnight falls (e.g. where the DayChange time is), may be different for different users so the window for day can be different from user-to-user.
