I grew suyo long cucumber last year and it
was not bitter at all. I really liked it.
I don't know exactly what causes bitterness,
some say uneven watering. 4 years ago I
planted straight eight cucumber and it was
bitter in my garden. Tried it a second year
with even watering and it was still bitter.
I think you should be okay with saving the
seeds, since it is suppose to be soil that
makes them bitter.
Suyo long is a great cucumber. If you grow
it next year and it is still bitter, try
a different variety. Yamoto is another one
that is suppose to be really good.