Always use double overhand grip (both palms facing your body).

Only switch to Mixed grip when your grip fails and you're not able to complete your lifts (or you know you wont be able to pick up the bar using double ovrhand).

If you always use Mixed grip, you won't develop your grip strength.

If you're a beginner, double overhand grip shouldn't be an issue. The weight is not heavy enough to make you fail.

YOu may succeed using double ovrhand up to probablay 2.2-2.3x bodyweight when attempting 1 rep max. You'll probably need to move straight to mixed grip when attempting anything above that (unless you have extremly strong grip). There are some who can double-ovrhand 3x bodyweight without straps.