Generally it will be significantly harder to get better quality talent in the early days due to the lack of business reputation and risks associated with joining a new start-up. "Cred" and job security are two things that engineers look for, and both will increase dramatically as you go from seed to B,C,D etc.
On the other hand Developers are more willing to take a pay-cut (in exchange for more equity) when they join an early stage venture. They understand that newer startups are financially constrained.
The vision for the company, it's mission and what it could become are super important in the seed stage. It's critical that you sell the engineers on this in your early recruiting efforts.
If you can make it to round A, it will be significantly easer to find quality candidates.
If you don't have a technical co-founder (and you probably should), you'll want to get a trusted dev to help you with your first 1-2 hires.
Finally, short of incorporating, I've got no idea how the legal requirements change in the US.
Employees are employees no matter how young or old your business is. Your investors will want credible justification for an expenses your business requires to grow. Once you reach a certain size (number of employees) you may be required by state or federal law to include certain benefits which change the cost of employees a little bit.