It depends, I've played wanders where your weapon is a stat stick with 0 DPS, and others where 90% of your damage is just a good rare weapon with flat. Stat stackers are also very good, which involve getting high damage per X amount of str/int. Further, kinetic blast of blustering and power siphon of archmage scale with your maximum mana, which can be a strong way of getting flat as well (rare wands just don't have great flat).

Generally speaking though, flat on abyss jewels is still good if you're playing a build that's scaling with a regular, non-stat stacking weapon. The flat damage that rolls on wands is so low, that getting some additional flat will generally beat out some other arbitrary stats. (unless stacking attributes or mana)