In general, I do not wear a tie tac with a silk tie. Then again, I usually don't wear a silk tie with my kilt, except for the band tie.

On the increasingly rare occasions I'm wearing a non-silk long tie (like my dad's old 100% virgin wool tie) with the kilt and need to wear a tie tac, I wear my Eagle Scout tie tac (mid-1980's style, that doesn't say "BSA" on it).