I use a stopwatch always when feeder fishing since seeing a Steve Ringer video six or seven years ago.
On stillwaters I'd be lost without one now as I find it a great discipline, especially as I am useless at guessing times! I do find that a pattern often does develop in bite times. However the stopwatch is purely a guide as instinct, observation and line bites all go into the mix.
On river I find it less important but do still use one at times.