It seems "TV" is always preferred when used as an abbreviation for "television" in informal writing, but is "tv" ever acceptable? (The capitalization seems to me to place undue emphasis on it.)
Upper case has the advantage of making it clear that the letters are an abbreviation rather than some kind of weird word. And it isn’t just TV. Upper case is often used for other abbreviations, even when upper case is not necessarily used for the full form. For example,
CD (compact disc)
DVD (digital versatile disc)
SIM (subscriber information module)
ATM (automatic teller machine)
ENT (ear, nose and throat)
GDP (gross domestic product)
RPM (revolutions per minute)
BIOS (basic input output system)
Lower case initialisms are found, but capitalising is by far the more usual. Lower-case tends to either be because it relates to a case where there are standardised lowercase abbreviations (kmph, km/hr & mph) or when the initialism moves toward being perceived as a word rather than its origin as an initialism (LASER and SCUBA becoming laser and scuba and even having back-formations like lase).
Using an unusual form like tv would give it too much emphasis to my mind.
The form teevee I personally dislike, but it is found and there's no technical reason to object. That said, it's been in use since at least the 1950s but nowhere near as much as TV, so it doesn't seem like it's ever going to be anything other than rare.
TV (caps) does give more importance than intended. I do understand the initialism, but argue that this is a word in transition and would apply the same principles as with a hyphen. Example: weekend began life as week's end, then week-end, and finally weekend, one word. The use of TV is not actually a initialism -- it does not stand for distinct words, as does RCMP or FBI or government departments; of FIMA; or any number of others. The original word is television (no caps); and no initials are even implied. It is a short cut used in the vernacular that by now should have devolved into 'tv', since that is the sense in which we use it. If the writing is formal, the term is television. Informal use begs for 'tv'.
I think 'tv' in lowercase would be acceptable in informal writing, because we say it all the time and people would know what it is. Capitalising it seems to give it some kind of title, which it does not have, and as you say, places undue emphasis on it. It would only have a capital T if that was the start of a sentence, as in 'Tv is boring today."
While I have wondered this question for a long time, I sometimes see in books "tv" instead of TV. I have also sometimes seen this: "T.V." and I have no idea why. But most likely you would want to put TV if you are writing something. I feel like you wont get criticized for putting "tv", but if you still dont know what is the correct way why dont you just write television. That is what "TV" stands for anyway ;)