For jigs I've always preferred soft lead but now that we're not getting it free anymore you really just can't turn down free stuff when it comes along.
I tested all my molds pouring just tire weight lead - most of them 1/16th and up pour fine with it, some don't. Ballhead no collar I can get down to 1/32nd with no problem. When I pour 1/8 and up I try and use the tire weight - and I'll pour just the four slots in the 1/32 ball no collar.
Lately what I've gone to and works really well for me is a 2/1 (2 parts tire weight - 1 part soft) and with this mix I can pour every mold I have. I don't smoke my molds or use a release agent and I have a very small reject rate.
As Happy said and others who pour below 1/32nds they want the pure soft lead.
Make sure to mark your lead!!! Put a T for tire weight, P for pure, 2-1 on 2 parts hard/1part soft. Write on your mold handles or a small notebook what type works the best in each mold. If you get an over abundance of regular tire weights (NOT ZINC) - go on some reloading boards and trade them for pure lead - the guys there are always looking for them.
If you need any more info let us know and we'll help you out.
Fatman