I have a shiny new titanium frame that I will be building up. I had the shop press in a headset and it looks like they used grease at the interface. Should they have used anti seize? I also need to press in a BB86 bottom bracket and I'm also wondering if I should use anti seize for that. I believe some people use a version of Loctite for press fit bottom brackets to eliminate creaking. Would Loctite act like an anti seize for titanium?
Generally speaking you shouldn't need either. The tolerances for press fit parts are made to do just that Press Fit! I have seen some mechanics use grease when pressing the headset but that is more so for ease of installation. It takes a lot more pressure to get it out than it does in. Being that you may need to change them at some point I would stay away from loc-tite as that layer will also effect the tolerances for said parts.
The best bet is to always check the manufacturers specifications as well.
In short I would use neither as long as you have the correct press tools.
I agree with @NateWengert . I'd add that anti-seize is used for... anti-seizing. If there are no seizing problems (many modern BB external shells are actually made of plastic, and plastic doesn't seize with Ti), don't use anything.
If you want to use something, use anti-seize (or grease, I'd recommend a specific Ti anti-seize though), but very little, or you'll prevent the fitting to be correct (I recall Shimano and SRAM specifying tolerances of less than 0.05mm, I can't find a link though), and you'll more likely get creaks and cracks in the long run... but most definitely, as @NateWengert said: if there's no reason to use anything, don't.