I play guitar and aspire to learn by ear. So, I've been using a metronome for scales and just strumming patterns, but I've heard it's good to play with a metronome for songs. I usually just slow the songs tempo down in Audacity and build up the speed. Also, If I'm trying to learn songs by ear wouldn't this be the best way so I can get the rhythm down and the feel of it, instead of playing to the metronome? I was thinking I could use the metronome after I learn the song just to see if I'm in time with it. What are your thoughts on this?
A metronome is simply a timekeeper. It does get used for technical practice such as scales and arpeggios, which, for exam purposes, need to be played in military fashion. Apart from that, there are not loads of songs which use consecutive notes from a key (scales) for more than a few notes. So being capable of running up and down scales to a ticking metronome has its limitations. (Any downvoters - please explain why you think this is untrue).
Using the metronome to keep in time is useful, if your sense of rhythm needs help, and having it on through songs is not detrimental. However - it can become a crutch to lean on, and you may not keep good time when it's not on. But, it tells you at what tempo you can comfortably play a piece - a yardstick.
Slowing down a song is a luxury that's not been around for very long, and it is quite useful, but what's wrong with just having the song running through your head (or even singing), at a slower than original tempo whilst learning ?
Also bear in mind that music is often NOT metronomic - it pulls and pushes, unless it's for dancing to, so being able to keep time like a robot won't necessarily make you a good musician.
They are often used in recordings - a click track - but that's more for reference later, when other tracks can be added without even being in the same studio, rather than to make sure the players keep time - if they couldn't would they be session players ?
To sum up, they have their uses, as and when a practising muso requires, but they are not sacrosanct. This may sound like I dislike them. Not at all. One of my hobbies is collecting metronomes. I know it's offbeat, it's not a wind-up, but what the heck.
If you play along with the song, I don't really think there is any need for you to use a metronome. You have the song to keep the tempo for you. If you try to learn the song while reading a sheet music or in a way you don't have anything to keep your tempo steady, you should always use a metronome.
It is the only way you can be sure you keep your tempo steady.