Bernie,For most of the time that my dad and I used that 6" Rikon jointer we were almost always using the wood we milled ourselves (poplar, maple, white oak, walnut, willow & some pine) and I never had a problem with the straight blades. After picking up a bunch of reclaimed black locust barn wood and starting to work with some exotics like purpleheart I did start having problems facing 6" stock with straight blades. It would get through that material but I had to go very slow and take incredibly shallow passes. I was thinking about spending the $300 on a helical upgrade but couldn't pull the trigger so I kept sharpening my blades and kept jointing at a slow pace.
That all changed when I hit my first buried nail in that reclaimed barnwood and ruined all three blades on the jointer. When that happened I spent the $300 and installed the helical head along with some new bearings. With that new head facing that black locust felt just as easy as facing poplar or pine on the old straight blades. The finish on the boards came out perfect and it even quieted the machine down. I've hit a few more hidden nails with this new head and taken no damage on the inserts.
I have invested in a nice little metal scanner that I should be using on all the reclaimed or urban lumber but I often forget that step before I start jointing or planing.
That would certainly make a believer out of you. I have never ran any reclaimed hardwood, but I'm certain there are a lot of unknowns.
There is a business locally that does just that, they sell wood that has been reclaimed and they sell a number of varieties. I ask them once about the possibility of an occasional nail and they said they find more that just an occasional nail and they have even found pipe partially embedded or driven into the timber. SO their procedure is to take a chain blade, which is like a chain saw circular blade mounted to a 5" horizontal grinder. they completely clean the board, all for sides then go over it and look for the shiny spots, cut them out and then run a magnetic scope over it like you do. They then then run it through the planer which does have a Helical carbide blade. It does produce some beautiful wood as I'm sure you do.