If you are not editing the image then yes you can leave it as a DNG. I don't think long term storage of camera raw proprietary formats is good idea.
A 5x7 at 360 ppi saves out as 13MB in 8 bits. Did you leave some layers and leave it in 16 bit for 85 MB?
If space is a problem aTiff with LZW compression can sometimes save a lot of space and is lossless.
Neil Snape photographer Paris printer reviews at http://www.neilsnape.com
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