Do I pluck with 45 or 90 (strictly across the string) degree angle? I'm talking about plucking, not placement.
Google shows various results of 45 and 90 and not all of them are strictly plucking, some are about hand placement.
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Both, and everything in between. Each person will develop their own angle, and will vary that depending what sound they want. And again, depending on whether it's fingers or fingernails (and the way they're shaped).
The 90 degree will give most force, so loudest, and clearest, but sometimes the tone and volume need to change, so more of an angled brushing will be used.
Since we all have different preferences, it's best to experiment to work out what suits you, where your hand hovers best, and how your fingers work. And, of course, what angle you're holding the guitar at.
For expressiveness, whatever gets you the tone you want.
However picking at 45 degrees has some definite disadvantages mechanically.
Lower volume. If you're not plucking the string cleanly, it's not going to be as loud.
Possible string noise as your finger skids along the string slightly.
With your finger more "side-on" instead of "flat-on", there may be more risk of catching strings on either side.
Picking at an angle tends to mean you're hitting the string more with the side of your finger and nail, instead of with the tip of your finger and nail. This will tend to wear away your nail at the side, which isn't a good thing for your playing or your finger. The softer contact area will also reduce volume again.
Ultimately you're the player and you can do what works for you. Just be aware of the potential issues you may face.