Yeah...it's up to you to decide what needs to be adjusted.
I'll usually get a feel for the song at the writing stage, and even at the early point, I will count out the BPMs so I know where I'm at.
Then as I further write/develop the song, I'll probably end up strumming/singing through it a millions times while working on it, and there are times when I change my mind about the tempo as the song develops...and sometimes it's maybe 1-2 BPM's up/down difference, and it either drops into the right groove or sounds weird.
I will always try the song out at a variety of BMPs from my original idea just to confirm the best choice.
Now...AFA guitar parts and all that other stuff....I don't write songs with anything other than the vocal and melody in mind.
I mean...I may have some ideas for what I'm going to do with the backing tracks, but rarely do I allow a backing track to drive my BPM choice.
IOW...the songs I write are about lyrics and melody...not licks and supporting stuff.
Of course, if you start off with some signature guitar lick, and it only sounds right at a certain BPM...well, then I guess you have to write the rest of the song around that.
Again...your song, your choice...but don't be afraid to depart from the original ideas if/when you run into conflicting situations...though you always want to keep in mind your original intent, and what it is about the song that you feel is the primary focus.
I have re-written lyrics to suit a more drastic change in tempo if I felt the tempo change makes the song come alive.