First time tiling a floor, I have hardibacker down throughout. Do I need to tape and mortar the seams?
Yes. And use the alkali-resistant CBU tape.
One tip, which might help or hurt - when you tape ahead of time and let the thinset dry before tiling you can get some bumps among the tape and thinset. These are less of a problem with big tiles which sit on a deeper thinset bed, but can be a pain if using small tiles on a shallow bed. You’ll want to keep a rub stone handy when you start lays by tile. Like here: I was not able to get a smooth finish on a shower pan, how can I smooth it out?
The other thing you can do is tape as you are laying the tile. This avoids the dry thinset bad tape bumps, but adds one more thing to the various things you’ll be juggling.
I can’t tell you which method is better, but you def want the tape so you’ll have a rigid substructure to prevent cracks. If it were me, and I wasn’t in a hurry, I’d tape ahead of time, being very careful to keep everything nice and flat.
Over HardieBacker, it dries out fast, in an hour or two. To help keep a smooth joint, take a grout sponge and wipe the seams down first with water. This will stop the backer from sucking the moisture out of the mortar too fast. Don't forget to put mortar under the backer, also.