It depends on the extent of the bathroom renovations. If the plumbing is in good shape and you are not moving the fixtures then they probably will not need to open the ceiling. If you are moving fixtures then they probably will need to open it to run new lines.

Sometimes I've opened ceilings below for shower leak remediation/repair.

For gut job bathroom remodels, for the most part, no. The ceiling below remains untouched. If required I'll open up the bathroom floor to the joists, do what plumbing adjustments need to be done, then install new subfloor/underlayment/what have you.

I just finished a remodel and the only reason I needed to access from the floor below was because I was changing the tub/shower to a larger walk in shower and a shower requires a different drain than the tub/shower.

The bathroom was demo'ed to the studs but because the toilet and the sinks were in identical positions they just easily used the existing drains and pipes for those fixtures.

When you are talking to the prospective contractors, I would suggest that you ask them specifically why they need to open up the ceiling or not so that you can evaluate. You might be receiving different input because you aren't really providing them with firm detailed plans at this point so they are just dealing with generalities.

I see, thanks! That makes sense. We would not be changing the layout at all, simply replacing the tile and the fixtures with newer ones. Good to know and I will use that approach with them