One of the best pieces of advice for writing music in Musescore or any other score editor is to compose with pencil and paper first, sitting at the piano if you have one. Computers are great tools for typesetting music, but don't always help with the actual process of composing. As Brian says, playback is a nice sanity check but if you rely on it you won't learn to hear the parts in your head. That said, some composers never leave their DAW so ymmv.
Whether the interplay of the lyrics and melody/harmony is important is something only you can answer. There's a long and august choral tradition of setting any old (usually religious) text to music without considering the words at all, but equally there are composers and songwriters who think very carefully about how both the sound and meaning of words and syllables fit with the melody, and how the words sung by different voices weave in with each other. Whether different syllables work better with different harmonies, or whether different sequences of syllables work better with different chord progressions, is something only you (and your ears) can decide.
By relying on a soundfont you are mainly missing out on a) the actual sound of the human voice and b) the consonants. There's a lot of auditory information in the consonants, for example plosives and sibilants are very different. If you really want to get into detail, vowels will sound different (and be easier/harder to produce) when you get to the higher notes. I would think primarily of the pitch of singers' voices rather than their gender myself, but countertenors definitely don't sound like altos. Again, only you can decide whether that matters.
You also miss out on the limits of your singers' ranges, their tessiture, and their lung capacity. Real singers like to breathe every now and again, and will thank you for bearing this in mind.
The good news is that the soundfont in your head takes all this into account, or at least you can train it to with practice and experience.
In general I would try to avoid giving a choir new parts too often, though it depends on the choir and your relationship with them. If you want to actively involve the group as collaborators in the composing process it can be fun and productive but it does change the dynamic between you, and tends to work better with smaller groups.
Remember no two choirs are the same. Think about what your choir are able to sing well, and also what they enjoy singing. Composing for a community choir has a very different set of constraints than composing for a professional chorus. A high school choir would be somewhere between the two, but depending on the high school it could be nearer one than the other.