I would FIRST check the terminals at the masthead and deck (even inside the roller furling). There were some Practical Sailor pictures recently on rigging checks which cause me to shudder. You can have a partial break which deforms (before it breaks) that would make for a very bad day.

Long-term, non-elastic deformation is not likely to be an issue. (i.e., exceeding the stresses in the rod or SS cable rigging to the point where there isn't an elastic rebound). Again,the areea I'd look at is the ends, terminals, (or even the spreader tips). In rare conditons the bow peak could have some issue, or the masthead could have become deformed.

As for sag on the forestay, we had about 3/4" of rod taken out of the forestay about 3 years ago when I had the sails redone. This winter, we cut another 1/2' off the forestay when we had the rig out to have all the fitting and rod checked (af few fittings that looked "suspect" were replaced).

The rule that our sailmaker applied was based on our J/Boats configuration (i.e., the position of the mast relative to j-dimension). He said we should see the first tell tails from the outboard position next to the helm.

If you have to stiffen the backstay tension to get the sag out in light air (say 10 knots apparent), then I'd talk to someone about it.

The downside if your roller furling doesn't allow you to take-up the slack is that the tighter you make it, the tougher removing the stay becomes. (We have a custom Harken that didn't have an adjustable forestay provision. The newer ones do. In our case, the tightening of the rod rigging on the forestay comes with cutting and re-applying a terminal.)

Not having "sag" is a bigger deal than you'd imagine. It made a huge difference when we took-up the sag the first time. The rigger and our North Sails buddy say this will really make a noticable difference in balancing the helm without over tensioning the head of the rig. We'll see.