The dressing or sizing manufacturer's place on linen - it's personal preference whether you like stitching on the stiffer unwashed piece or want to wash for a softer linen (softer usually preferred by stitchers who stitch linen in hand instead of in hoop or frame).
If you have creases in the fabric, particularly if you'll be using non-colorfast overdyed specialty threads that cannot be steam ironed or washed after stitching - you need to steam out or wash out the creases before starting your project.
If you wash - do not ever wring plain or stitched fabric.
In addition to Orvus there are now a lot of quilt and cotton heirloom wash products to look at. The only ingredient should be "Sodium lauryl sulfate" - and you can use those safely with cool water on colorfast fabric and threads.
Linen straightens nicely just tugging on diagonal without worrying about wetting down to do major blocking.
Not sure if threads or fabric colorfast? Put a snip in clear container of water for an hour and see.
Solid color Belfast or 32-count linens are likely colorfast (particularly name brands like Zweigart, Wichelt, DMC, Charles Craft, etc.) but never hurts to test a corner to see.
Rating
Washing linen by: Jane
I was advised not to wash some Irish linen before embroidering it, as it was "dressed" for easier embroidering and this would have washed out. I decided to do my project first, and to wash when finished. Needlework Tips & Techniques have some great advice on washing technique on their web site. Jane