If you're bothering to change your cl_interp manually you should always keep the cl_interp_ratio at 1 so you don't have to worry about it whatsoever. cl_interp_ratio is a leftover from old versions of the Source engine where they removed the ability to set cl_interp manually and you could only change your interp by changing your updaterate (or messing with the ratio). The server-side restrictions actually used to be perfect but they never added any more after giving back the ability to change cl_interp manually again.
athairusThe one thing I still don't understand is why people advocate for different lerp values based on classes.
The reason why soldiers use low interp is because your client-side projectiles are delayed by your interp amount (ex: cl_interp 0.3 would delay your rocket graphic coming out by 300ms). If you use high interp values as as soldier, your client-side rockets will not exactly match where they actually are. I'm not 100% sure on this though (haven't checked into it since 2009).
The way I approach my network settings is as follows:
cl_cmdrate and cl_updaterate should always be set to the server's tickrate. (66 for TF2)
rate should be set to whatever your connection can handle (50k+ is typical these days)
If you want a truly optimal interp for your own specific connection, then go play a few games in your usual environment (favorite pub server/the usual server you play 6v6 or HL at the usual times), record them locally with OBS or whatever other software and leave net_graph open the entire time. Go through your games afterwards and watch your net_graph's packets/second and look for the lowest amount you get (typically in big fights). Set your interp based on that value.
example: Bob had a night of games with his team, skimmed through it at triple speed and found that the lowest packets/sec he got all night was 44, and he had no packet loss. Therefore he would set his interp at 23ms.
The other network settings (ex: smooth) don't really matter that much for 99% of people so I wouldn't worry about it.