My experience with using the same SSID for both bands is that most devices will do the right thing, pick based on signal strength and will reevaluate that choice fairly often. However, a small number of devices (normally older or cheaper devices) will often stick to a band until they have no other choice.

That said, a weaker 5GHz signal can sometimes still be better or more stable than a stronger 2.4GHz signal since the risk of interference in the 2.4GHz band is often much higher.

I’ve never ever had enough of an issue to worry about it.