Mine is straight up/down. I get the LF (Low Flow warning) every so often, but confirmed with Fluval that they don't need to be at a specific angle. Its just placement in a flow area for the sensor which is at the top of the unit. They also stated that the heater will still function properly with the Low Flow warning, just as any other heater would in an area with limited flow. Even with mine, I don't know how I can get a LF warning since it is at the output of my filter and has a ton of flow at that particular spot.
I do love those heaters though.
There are tons of complaints that these heaters don't work properly when they are stood vertically exactly like you described LF warnings and even taking forever to heat the water. I've seen fluval comment to turn the heaters to a 45⁰ angle and presto, the water heats normally. Here's a typical comment I see all the time:
This can be avoided by placing the heater at a 45⁰ angle near water flow. I've tested this myself by placing the heaters vertical and I can tell you without a doubt there's a huge difference in heating time or even just keeping a stable temp when the ambient temps are more than 10⁰ different.