I would say though until you know your preference it's better to err on the side of underbooting.
I do agree with this, to a degree - in terms of risk of injury it's safer to be slightly (and I do mean slightly, absolutely not significantly) underbooted for most skaters. By underbooted I also don't mean skating in boots with broken down support - that is not the same thing.
That said though, I think most beginning adults actually do better being slightly overbooted at lower levels, but not to an extreme degree - mostly it's because the skates appropriate for an absolute beginner will be too little support for an adult skater once that same skater's skills have progressed just a small amount - in this situation, I comfortably recommend lower level freeskate skates for most adult beginners - just because if their skills do progress quickly, they won't find themselves needing new skates within just a few months, and those same boots are not likely to cause any problems with skill development if they are slower to progress - if anything the saktes will just last longer, which for an adult with feet that are not growing, is a good thing, provided they were fit correctly.
Kids on the other hand, really don't skate well when they've been overbooted, even by a relatively small amount - I've seen kids at all levels who simply can't bend their ankles at all, and never really break in their skates before they outgrow them, and it's super frustrating for the kids, and it's frustrating to watch as an instructor because I know they'd be able to skate better in different skates. I've been SUPER careful with my daughter because of this, so she's not struggling to bend her ankles. She's on her 4th pair of skates in 3 1/2 years, currently in freeskate 1 and skating in Pro Teri's, which has worked out really well for her so far. I would have opted for another pair of Jackson Elle's if we hadn't found her current boots in the size we needed for $80 on ebay - only because I couldn't see spending full retail on them ($450+ just for the boots), not because they're not an appropriate choice. The pro teri's are more similar in stiffness to the Jackson Freestyle which would have been fine for her as well - the Elle is just a great choice because it's a bit softer and has nearly no break-in time - in my daughter's case I wouldn't have expected her to break them down before outgrowing them, so opting for a softer boot would have been okay - in another couple of years when her feet strop growing so much, that wouldn't be the case.
All that said, I am "overbooted" and I know it, but it's what works for me - it doesn't work well for everyone. I have a history of breaking boots down quickly (and I mean in 6 months type of quickly - and I'm not in a position where I can or want to be replacing my boots every 6 months - as it is, I've consistently replaced the last 2 pairs after about 2 years, which I am okay with) and my stiffer boots are NOT causing me any kind of problems (like the inability to bend ankles, poor edge control, etc), but that won't be the case for everyone in a similar boot.