Sinew shrinking doesn't give the bow much reflex at all. Most of the gained reflex comes from tying the string between the tips and increasing the reflex after each layer.
The sal should be straight to begin with.
Adam explains the reasoning behind not having reflex in the sal it is to do with separational forces between the layers.
You can have reflexed sal sections, as many many bows back in the day had. Separational forces are real, bigger with thicker limbs, but doable with good glue up work.
Crab bows, korean bows, tatars and turkish flight bows area all believed to have almost tip crossing reflex before sinewing.
Sinew reflex is nice, but what really makes the form is the wood core and horn. If sal sections are straight before sinew, eventually the bow will also be straight in these sections after tillering. All these bow types i earlier listed, had quite generous reflex when off the string. Well, my bows either haven't been reflexed this much, but in the future they hopefully will be.
That said, for a beginner or for a really heavy 150+ pound bow its good idea to have the bow less reflexed. It raises the possibility for a successfull bow greatly.
Good luck!
