I'm not a professional arranger, but I did have the opportunity to study with one. With regard to written bass lines, I think his advice would be that you have to know who you're writing for. If you want your music to be playable by students you'll want to be modest with instrument ranges, write out baselines, etc. If you're writing for your own band or studio players, they know what to do.
As a piano player, I've seen a good number of piano parts from the published versions of Nestico arrangements. I don't know what Nestico actually handed to Count Basie, but Basie sure isn't playing the notes written in my part most of the time. I'd guess Nestico or someone at the publishing company wrote the piano parts years later so that high school bands could buy and play the charts.
My arranging professor had a few interesting stories about writing readable parts. For example, he mentioned a chart once someone had arranged for a few big-name jazz guys to play with a pops orchestra. The jazz guys know how to swing, so they had a typical jazz part in 4/4. The orchestra guys, on the other hand, don't really know how to swing. They got the same part written in 12/8 with triplets instead of swung 8ths.
The first step in writing playable and readable charts is deciding who needs to be able to play and read them.