If you’re upscaling to 5.1 just for the purpose of including the LFE, I wouldn’t bother as it’s a lot more trouble than it’s worth. Maybe mix in the LFE @ -10dB.
Or there’s a more elegant way of delivering stereo+LFE - direct bitstream configuration.
AC3 & DTSMA can support 2.1 natively, and encoders are readily available.
If you’ve actually heard what my 5.1 mixes sound like with the LFE, it’s unlikely you’d say it wasn’t worth it.
As for encoding in 2.1 format, I did try that a long time ago, but I won’t do it again. 2.1 is out of spec for AC3, and only non-Dolby encoders allow for this channel configuration. DTS does allow it, but receivers can be quite unreliable as to whether they’ll actually play it back properly. Some of them will, but others will treat it as a stereo signal only and ignore the .1 altogether. Upmixing to five channels and combining with LFE in order to create a standard 5.1 format was the only way I could reliably obtain both the surround audio and the enhanced bass response together in one mix.