At least for Digimon World 1. A Digivolution guide is handy but not needed. At least for Rookie -> Champion.
As long as you know about care mistakes at least. If you don't and you overwork your Digimon a lot, you're fucked.
I always kept my Agumon's stats even which got me Greymon a good 9/10 of the time. The other time was when I train brains too high and get Centarumon.
It is funny however making your way to Toy Town on a new file as a Numemon, getting inside and then getting Monzaemon. (happened on my first or second playthrough when I was first played it as a kid).
I'm pretty sure I'm not the only one who has done that without meaning too (aka not a challenge run where someone on purposely gets a Numemon to go through the game with
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)
For other Ultimates though, yeah best to use a Digivolution guide otherwise you're likely to waste a lot of time as you try and try and your Digimon ends up dying from old age (unless you're lucky and get Phoenixmon from the ashes of your Birdramon who died in battle - that was a fun day)