The other day I was given these taxi instructions:
"Taxi runway 27R via A, D, follow the Merlin."
Taxiway D to Runway 27R crosses runway 35. Is asking the ground controller if I am cleared to cross runway 35 unnecessary due to the "follow" command?
No. you definitely should have asked if you were cleared to cross runway 35. In fact, it should have been explicitly stated by the controller, per the 7110.65U, 3-7-2.d:
d. When an aircraft/vehicle is instructed to “follow” traffic and requires a runway crossing, issue a runway crossing clearance in addition to the follow instructions and/or hold short instructions, as applicable.
“Follow (traffic), cross Runway Two-Seven Right.”
“Follow (traffic), cross Runway Two Seven-Right, hold short Runway Two-Seven Left.”
Not getting that crossing clearance definitely would have been an operational error on the controller's end, as well as causing some issues on yours for crossing the runway without explicit clearance.