Since version 5.0 there is a general setting to set billable by default.
https://www.manictime.com/releases/Version-v5-0#setting-default-billable

I hoped this general setting would help, but unfortunately it doesn't. Because now all my non-billable tags are billable too. And afterwards I have to remove the billable marks. It takes me too much time.

It is necessary to be able to set billable default PER SEPARATE TAG. I hope this will be implemented. Thanks.

ability to set tags as defaulting to billable rather than defaulting to non-billable. any new tag should be billable unless otherwise opted out.

+1 This would be a big time saver for me as well.

Big +1 here too.

I tag the whole day and then have to go back and edit each tag that should be billable to make the best sense out of the data. Then I have to (manually) enter into our timekeeping system (eBility). Ugh.

It's easy to filter tags by date. It would satisfy me if I was able to filter and then update multiple tags to be billable using the existing filter window. Currently, the filter window only lets me make a new tag which is not useful in my circumstance.

Marking a tag (or tag combo) to be billable by default would definitely be a big help since we plan to implement the QB export facilities and QB uses the billable/not billable fields for invoices.

Big +1 from me.

You could have a scenario where you want to make all tags which come from (or match) the jira tag source to be billable. (Full disclosure: billable isn't something I use, since billability is determined in the downstream timesheet app, not in manictime)