(Originally written as comments, but moved to an answer as it got too bulky)
First of all let me say on my living room setup I now use a mixer (and have voted for Todd's answer). But to answer the question and show it can work, here is what I used to do:
I have a standard (if old) domestic amplifier and speakers. The amp has around six or seven inputs, if you include the "Tape" inputs. (Rotel RA-940BX amp, Mission 731 speakers on decent stands; all reasonable mid-range domestic hi-fi equipment at the time.)
I used to have various sources, all connected to Line inputs on the amp. Guitar preamplifier, a keyboard or digital piano, some other synths, and my computer. Some of these had only headphone outputs, so I used long 3.5mm jack to RCA/Phono leads for these.
Simply select the input you need on the amplifier, set the volume you need, and blast away.
Possible disadvantages of this method:
- You can't use the instruments to play along to pre-recorded music - there is only one source active at a time. (This is why I now use a mixer instead.)
- When selecting between sources, you'll probably find some are too loud and others too quiet. So having to tweak the volume control all the time is slightly inconvenient.
- Lots of ugly wiring (though it can all be hidden around the back of the amp I guess.)
- For electric guitars: some will find an electric guitar preamp's headphone output dull and uninspiring to listen to. I'm perfectly happy with mine (a very old Marshall 9004 transistor preamp and sometimes some home-made electronics using steep treble cutoff filters) but this is very subject to your taste. Any choice you make here is putting at risk your money and your fashion sense! (That was meant as a little joke; I don't care if people disagree with my sound, enough said.)
- Also for electric guitars: for decades I've heard comments from hi-fi experts that running an electric guitar, even on clean settings, through normal domestic speakers would ruin them. Well mine have lasted at least twenty years at normal "living room" volume; the only damage I can find is lots of dents from being clumsily knocked off their stands from time to time. I can only say doing the same is at your own risk, and don't blame me if any equipment is damaged or the world ends.
So - subject to one's taste and to whether one believes speakers will be damaged by certain instruments - running sources directly into a domestic stereo is fine.
But finally - as I stated at the start - for home use I now prefer Todd's solution, being an electronics hobbyist I built a simple mixer and now can play along to music, or watch films on DVD and plink away at those Hans Zimmer soundtracks on my piano, synth, guitar or electric ukulele. That's far more useful than selecting a single source, to me. Are you not entertained?