A lot of cruising boats do not have telltales on the luff (area) of their mainsails and often lack outhauls to adjust that shape of the sail.
But other tools do also adjust this shape, like the combination of your traveller and mainsheet, especially higher up your mainsail.
Using telltales closer to your luff will give you a good sense of how it is shaped related to the airflow that the foresail is providing for you. It will tell you if you need to change things to get the most powerful part of your mainsail working better.
Now here’s the point, the traditional leech based telltales only tell you that airflow is adhering to the sail, NOT what the Luff shape is doing to that airflow and you basically want this luff shape as fat as possible until it starts to disturb airflow.
The only way you can do this is see the airflow there, at the most powerful part of your sail, so add telltales to your mainsail closer to the luff.
Now a word about placement. Luffing and disturbance occurs closest to the Luff first and then later further back. If you want twitchy information that will allow you to keep optimum tune at all times… put them closer to the luff, at least three places up the sail (reef points is a good place) or if you want to have a slightly more cruisey view of your Luff airflow, bring them back about where the luff curve is starting to ease into the body of the sail.