It depends on your business model. If you’re selling expensive enterprise software, 100% you should customize the demo and presentation for each customer. Minimally use each prospect’s data/business/market vs. something stock. The best pre-sales engineers I’ve ever seen make this a habit and generally build their own scripts/utilities to make spinning up new custom demos cheap & easy.
If your business model is more transactional, then you probably should have one killer demo that you can repeat over and over. In that case, you’ll spend your customization dollars on the framing presentation and demo path rather than the software/data/configuration of the demo.
We do both at Salsify, since our business model ranges from transactional up through global enterprise deals.