I think something is a money saver if it's better than your next best option. In your bring your lunch example if the next best reasonable option is buying a $5 lunch every day then bringing a sandwich is indefinitely a money saver.
In my mind it is irrelevant that it's not saving you more money from what your situation was in the last week/month/year you were using that tactic, only that it's saving you money over another reasonable option.
I definitely think a person's preferences have to be taken into consideration.
1. "Reasonable" is in the eye of the beholder
2. "Savings" over the next reasonable option may be negative
3. Saving money vs. wasting money are opposite sides of the coin, and it depends on your point of view.
Take for example - the post above regarding coffee. Apparently bringing coffee in a thermos is a money-saver for that poster. But if I forgot to bring coffee to work,
my next best reasonable option is to go without coffee. To me, home-made coffee is a
luxury, not a money-saver.
(Back in the days I worked full time) I came to
prefer bringing my lunch to work. It was the best option! Tastier, healthier, cheaper, and faster than a cafeteria or a restaurant. To some extent, I considered the occasional time I bought lunch in the cafeteria to be a money- (and time-) waster.
Preferences may change over time. I would agree that as long as preferences don't change, a money-saver continues to be a money-saver.