Genuinely one of my favourite threads in this room, because nobody forced the two languages into one. Let me give the answer I'd give a new practitioner who asked me this in person.
The solar plexus focus is both traditional and functional, and I'd resist any answer that says only one. Traditional, because it's the agreed centre we all practise from, and shared geometry is what lets us compare notes at all — if everyone held the mudra wherever felt nicest that day, this whole room would be comparing different things. Functional, because, as Juniper and Milo describe, the solar plexus really is where "holding a field between the palms" wants to sit: the trunk's relay point, the natural midline between chest and belly. And as LucentMoth says, it's meaningful — it stands for the balance point between Primary above and Counterplane below.
So: is it essential? For the practice we share here, yes, keep to it. Is your drift upward toward the heart a mistake? Not a moral one — but I'd treat it as posture to correct rather than a preference to follow, at least until your hold is very stable. The one thing I'd say firmly is don't force it. If you clamp the hands to the solar plexus by willpower, you lose the very ease the mudra needs. Set it there gently, let it settle, and if the heart keeps calling, note it and bring it back rather than fighting or chasing it.
As ever — describe what you feel as sensation, begin with grounding, and don't turn a placement preference into a doctrine either way.