Let’s first look at your preview. Is this in an external monitor or in the Premiere preview window ?
The Premiere preview window will never look as good as the export, since, even at full quality, it will be rasterized. So, either do a small uncompressed export, or take a look at it in an external monitor for a true quality assessment.
The second thing has to do with DV. If you ARE working in DV, bringing a title over footage will force a new compression to DV of that entire clip.
This change may not be drastic, but it will be noticeable.
Now, unless you are exporting to DVCAM / MiniDV, that doesn’t matter, since it will not reflect the final export; it’s only compressed for preview.
Hope that helps,
Vince