I think a lot of us really envy that you have the OPTION to use Slack. I wouldn't think of this as a bad thing at all. If you've on a good, high period of speaking well, go talk directly to them. If you've been having a tough day or the stuttering has been bad recently, use Slack. No reason to put too much thought into this. All in all, enjoy having the option of using Slack, but mix it up.