When making citations, I've seen both styles: with the tie/non-breaking space (~\cite{refnm}) and without it (\cite{refnm}). There are a few questions & answers on this topic in the more general sense, recommending reasonable things like a tie before a \ref. However, \cite never seems to be mentioned in these lists...
I happen to be mostly concerned about numeric styles like IEEE, which I think is unlikely to produce bad line breaks if a tie is used, but what about other styles, and is it really recommended?
Interestingly, the IEEE Editorial Style Manual only says the following about references, which I think a few other guides have turned into "should be one the same line" (I'm pretty sure they mean as opposed to super/sub-script).
When they are, they appear on the line, in square brackets, inside the punctuation. Grammatically, they may be treated as if they were footnote numbers [...]
I did look at Why should I put a ~ before \ref or \cite?, and I thought that \cite deserves a separate question - I think it's pretty different case from \ref. I guess I'm looking for a more stylistic opinion.