I've kept my flex conduit runs pretty straight, and have only used fiberglass rods in them. Not any real issues, but adding cables one at a time can be difficult, with cables binding to each other. If you plan on many future cables, piecemeal, I'd say try to use multiple runs of conduit, for a basement -> attic chase.
Mostly, a conduit to a TV location will be used for a single future HDMI cable (which han't been invented yet). Run cat6 x3 + RG6 x2, plus a current HDMI cable, outside of the empty conduit, and I think you'll be set for 95% of needs. If you think you may need more, add a couple runs of cat6 outside of the empty conduit (cat6 x5 total).
I feel that residential conduit is for futureproofing, not for current needs. Run the cables that you know you will need outside of the empty conduit.