STOP IT
JUST STOP FUCKING DOING THIS
YOU DO NOT SAVE YOURSELF A GODDAMN SECOND

1. The difference between "No sentry" and "Level 1 sentry" is FAR more profound than the difference between "Level 1 sentry" and "Level 3 sentry". It's a glass cannon intended to stop weaker or unsuspecting enemies that have gotten past your other players - just having a level 1 guarding the point early on can stop, or at least severely slow, a spy or scout capture. If a class doesn't notice a sentry before they walk into its range, even a Heavy can be taken down by it.
On the other hand, choosing that "It's either a level 3 sentry, or I'm useless" could NOT be more wrong. There are no situations in which "a level 1 sentry would be useless, while a level 3 sentry would be vital". Upgrading your buildings is simply icing on the cake to use your extra metal for.

2. Ammo in the field is usually plentiful! Goldrush is a particularly good example, with two full-resupply ammo boxes in walking distance, but the same principle applies on other maps. Know where some nearby ammo grabs are, and you don't need to be chained to the spawn room.

3. Dispensers are super useful for your teammates! It's beyond tragic that engineers waste 400 metal on machine guns and rockets, when a quarter of that could be used for the team's anchor. Dispensers heal everyone even when your medic is dead, they enable half the classes to spam their weapons in all directions with no regard for ammo (from grenades to spy cloak) and ensure that any good killers on your team can survive several kills and resupply for more. Be especially sure you don't place your dispenser in a hard-to-reach spot, such that anyone ducking for cover will immediately notice your kindness.
As an added bonus, teammates clustering around your dispenser are far more likely to defend it from spies, ubers, or whatever other nasties intend to destroy your hard work.

Music is from ポップンミュージック 8 (Pop'n Music 8); its use was inspired by CosmoDrazi's Source Filmmaker video "Goldrush"