Yes, you can. You should do it especially if you imply to not have been in the cloister/Gith creche yet. You will appreciate the xp. I was lvl 6 at the end of the Underdark (although I lost some content by the "Grymforge NPCs are gone" bug (?)), and lvl 7 at the end of the cloister/creche content. Nice to have in the shadowcursed lands.
To split hairs, technically the Underdark is part of Act 1, I presume, as it was available in the EA.