Erm, not necessarily and it depends on how much. A healthy person can compensate for some drops in blood volume, but only up to a point. If you're lost in the desert while hiking and run out of water, your pressure will eventually drop.
It would be hard to know which of a person with ME/CFS's compensatory mechanisms for low blood volume are still working without medical work-up. And, even if they do still work to some extent, many people with ME/CFS find hydration to help their blood pressure and orthostatic symptoms.