Can I upload all my tax rates from a spreadsheet?

Yes, if the Import / Export plugin is active. an Import button (plus Export CSV once you have rates) appears next to Tax Rates on Settings → Store Settings → Manage Tax.

1. Click Import. The Import / Export — Tax & Localization page opens on the Tax Rates tab.
2. Click Sample CSV to get the format. The columns are country_code, state_code, city_code, postal_code, rate_tax and tax_name.
3. Use location codes, not names, and * (or a blank) for "all" at a level.
4. Upload your file and follow the steps.

Re-importing updates the rate for the same location instead of adding a duplicate, so you can export, edit and import again. Rows with an invalid rate are skipped.