I've covered engraving NFA receivers for SBR registration in this article. However, if you specifically talking about an AR-15 lower here are some things you should consider.

After talking to several people dealing in NFA items and my local ATF agent, here is the reason for engraving. Since you are technically "manufacturing" the SBR buy putting a barrel shorter than 16" you are required to engrave in the event you wish to sell the gun to someone else. I engraved my USC/UMP conversion in the event I ever wanted to sell it.

The AR-15 lower receiver is generally a cheap component. You can always sell the upper receiver separate from lower. By keeping the registered lower and not selling it eliminates the need for engraving.