Hiya!
As Lil Shenron said, plus: When asking about 5e questions, one thing that should one should always keep in mind is that 5e
requires DM involvement in many, Many, MANY aspects of character mechanics choices. The DM in 5e is basically a "built in catch-all rule" for
everything in the game.
Basically, if a rule can be interpreted in more than one way, it requires DM involvement. In the OP's original question, it is obvious that he is interpreting a "rule" from a "non-stated assumption". In other words, because the rules don't explicitly state something, he is arguing that he can "automatically" do something. In fact, he should interpret that as "Oh, I think normally it is assumed that all spell slots are filled, but maybe not... I guess I should consult the DM" (or, if he is the DM, make a ruling and keep on playing).
For me... in a "Generic" D&D world, I'd say you need to do all or nothing. Sorry, no "filling 8 of 10 slots then waiting to do the rest later in the day". In a different campaign world I may rule differently. This is the beauty of 5e (and older versions, like BECMI)... how the 'actual' rules of the game are going to be used/interpreted is up to the individual DM. This allows for a virtually limitless amount of different campaigns, DM's and play styles. Yay!

I always hated saying "In my Pathfinder campaign, I restrict paladins to being only human"... and then almost instantly getting pounced on by rabid power-gamers, rules lawyers, or "character build optimizer's" about how I was a bad DM, or otherwise "sucked" because I somehow "didn't want players to have any cool, fun stuff".
^_^
Paul L. Ming