as for general bit rates... I tend to see it like this on most equipment (like speakers and typical headphones) in that 96kbps (CVBR) is going to be at least 'good enough' and it's very efficient. I could maybe see 128kbps or so as a bit of a safety buffer but much beyond this lossy encoders lose their appeal if you ask me. because while I get most around here have a tendency to obsess over tiny sound differences, in the real world, they are likely quite small to where the average person ain't going to notice it. even if they do, it's not something you will likely notice in general when just sitting back and enjoying ones music.
because I figure if someone does notice the smaller stuff and it bothers them, at that point, if possible, they are better off using a lossless format like FLAC since I think the whole point of lossy audio is to retain a high percentage of the sound quality but at pretty much smallest possible file size that will suit many, if not most, people well.
p.s. in regards to TVBR vs CVBR... CVBR might be a touch safer than the default TVBR on sound quality if one has to choose, but even here we are splitting hairs. but a slight negative with CVBR that favors TVBR overall is it seems CVBR tends to have, on average, bigger file size vs TVBR. so this is a really small thing that many might not care about. but if you do, then you can make a choice here if you would rather have the smaller file size(TVBR) or 'maybe' a touch better sound quality(CVBR). but when using 96kbps, since I feel this is the lowest bit rate I would suggest people use on AAC(AAC-LC), I tend to be on the side of caution with sound quality and opt for CVBR. but higher bit rates (say 128kbps or more) one might as well opt for the slight storage space savings with TVBR.