Putting a cat to sleep before it "suffers"

As mentioned in previous posts, my cat has masses on his intestinal lining and the vet strongly suspects cancer. I found this out on October 27. I have since decided to keep my cat happy at home and chose not to opt for exploratory surgery/biopsy/chemo. He was 11 pounds in June and was 8 pounds last month. He continues to eat but has his good days and bad days. I don't believe he has had a bowel movement in a while and this past weekend he went into the litter box, like he wanted to try to have a BM, and then proceeded to get out of the box and lie on the floor next to the box. This is new - he has not done this before. He did the same thing today when I went home for lunch. It's like he's trying to have a BM but can't. He also has slowed down in his walking, especially up and down the stairs. He often wobbles too when he walks. I called the vet with these latest concerns and I have an appt. tomorrow morning to have him checked out. If she feels it's "time", then I will concur and put him to rest. I quite honestly don't think he's got months left since he is so thin. If she tells me he may have "weeks" left, part of me tells me to put him to sleep at the appointment to avoid having him get worse and having to see him suffer. I often hear owners say they put their pet to sleep "too late" and when the pet is suffering. I don't think there is such a thing as "too soon"...well, not for my cat but for me. It would break my heart to see him really suffer if he's not already. Has anyone made the decision to put a pet cat to sleep before any significant suffering occured? Sorry if this seems rambling...trying to post all my thoughts in a long paragraph is awkward. Thank you.