This is a topic that came up in our recent campaign.
We agreed that just like a person doesn't walk around repeatedly holding their breath in case they fall in water, a fighter doesn't walk around with his weapons drawn, my Warlock doesn't walk around expending the (even relatively little) effort to keep recasting a spell on herself 'just in case'. If nothing else, calling upon my pact over and over without any real reason is a draw on my patronage.
If on the other hand, we see combat coming, then I cast the spell as we ready (just as everyone is able to draw their weapons) and I roll 1d4+4 (once).
edit: For example, if we're walking into a magical tower where the Lich was last seen, I'm casting False Life. If we're strolling through the woods on 5hour march to our destination, I'm not recasting it regularly.
Why not always start with 8 hitpoints? Simple. I only know the spell worked. Not how well it worked. Whatever my first successful cast gives me, I have no reason to believe I can do it better by repeating the process.
Thanks to @IcyFire's comment. We play that PCs don't know the numbers of their stats. We know a clean, hard hit vs a glancing blow. We know when healing has made a substantial difference vs when it only seemed to stem the bleeding. So I acknowledge that this part is probably not relevant to all.