There's risk with Discover in that if it doesn't qualify as 5%, then due to their tiered reward structure it will only qualify for 0.25% (unless you've hit the $3500 or so spending for the next tier, that is still less than 1%).

On the other hand, if you try it on days running those 2% promotions (which can be stacked), the reward can be significant. For example last month (Aug 17th to be precise) it was earning 9% on the first $250 spent at gas stations, 7% for the next $1250. I'd try on a day when one of those additional promotions can be stacked (assuming there is a promotion some time like that next quarter).