I've seen more than one optima burst into flames if it's given too much current. Jump starting them can often deliver excess current as the alternator maxes out once it's fired off. Might not happen at idle in a classic car, but once you give it some revs it might get dicey.
Trickle charging to a nominal voltage is the safest bet, but I've heard from others that they've never had an issue with jump starting an optima either. I personally watched one in a hyudai santa fe melt in on itself and catch fire when it was jumped in the autozone parking lot, and have seen 2-3 that were placed on fast chargers in an auto parts store that did the same.
If you decide to jump it, be ready to key it off if anything starts to smell bad or smoke, and I wouldn't wait for a bad smell to turn into smoke.
Edit to add: if it was alive enough to crank before it started to click, it's probably OK to jump. The catastrophic ones I witnessed were super deeply discharged.
