So would preallocating buckets be helpful or hurtful here

If you know the final size, preallocating to that size will certainly do no harm and speed things a little.

Hashes get 8 buckets to start, then double in size each time as they fill. When the doubling occurs, the key/value pairs have to be copied from the old to the new hash which is a relatively expensive operation. Moving stright to the final number of buckets avoids that.

Of course, even if you pre-sized the hash to 2^18(262144) buckets, there's no guarantee that will be the final size. It could be that hash collisions at that size will prompt another doubling dependant upon the keys.

You could go straight to the next size up directly to be safe, but then you are potentially wasting memory. Which if the ultimate criteria is speed may be acceptable.

But without knowing the OPs actual criteria there's no way to know for sure.

If runtime performance is his only goal, using an array rather than a hash would be twice as fast. And despite being sparse, would actually use less memory.