In my experience, hanging a bike up by its front wheel can cause migration of air from the reservoir into the active circuit over time, but generally speaking it needs to be overnight or longer. My current bike was bought SH and the front brake would go spongy over a couple of days. When I investigated the reservoirs were nearly empty. The chap I bought it from claimed to be an engineer...
When bleeding I always remove the cap and diaphragm completely (even on Shimano brakes as I don't trust the funnel). After eliminating all air from the system, I deliberately over-fill the reservoir to overflowing. The diaphragm then needs carefully laying over the top, avoiding trapping any air. If you're not displacing fluid out of the reservoir at this stage then there isn't enough fluid in it - use a rag to catch the overflow. Replace the cover. Often, there's now too much fluid in the system and the brakes drag a little. Just open the bleed nipple slightly and gently squeeze the lever to eject a little - on no account release the lever before closing the nipple, or you'll have to start again! Remember to thoroughly clean the fluid off the brake lever!
No brake I've dealt with this way has ever required a rebleed. The fluid should last for a number of years without requiring attention.