No, they're both used equally as much in both pos. and neg. sentences. But they translate to English differently.
- Ještě píšu dopis. - I'm still writing a letter.
- Ještě nepíšu dopis. - I'm not writing a letter yet.
- Už píšu dopis. - I'm already writing a letter.
- Už nepíšu dopis. - I'm not writing a letter anymore.
For Czechs who learn English, it's baffling why English expresses these so "inconsistently".