The answer is "whatever the DM says". If the DM says you can re-roll it's not cheating, if they say you can't (and you do it anyway) it is. I've run campaigns with point buy and rolls in 5e and honestly I think it depends on the campaign which is best. Some of the best characters and roleplay come out of the wonky stat distributions rolling provides. Personally I let people re-roll if they get a truly terrible array and make the re-roll if they get a truly god-tier one that I feel could legitimately upset play. We have an excellent int 6 barbarian in the party at the moment and the player really leans into the character not understanding complex things without either being suicidal or a detriment to the party. I ask the players to recap the previous session as their character would and they all make this player do it because damn the man is funny.

I can't recall if it's in the original games but I'm sure in the Beamdog re-releases you could actually dig out the times a character had had their stats re-rolled from one of the .ini files or something else easily viewable in text readers.

I do think if the option is put in (which I think it should) there should be some limitation for multiplayer available as a toggle (the more the better but disable/one roll only would be the obvious limitations). Raw stats in 5E aren't as big a deal as they were in AD&D/BG1or2 when they were tied to things like learning spells, amount of spells per day etc. But raw 5e has some surprisingly rigorous adherence to expected bonuses in your primary stat, the AC-to-CR of the average monster and the expected player to-hits is almost always 65% (the game "assumes" you take an ASI in your primary stat at 4 and 8 to hit 18 and 20).

Even if stats don't affect as much as they used to they'll still be a noticeable effect with someone whose has a higher bonus so there should be limitations available to the host player in MP should they wish to use them. In SP let the players do what they want within reason, I like rolling stats and would happily roll for a few minutes to get an interesting array of numbers, I don't plan on sitting there for 2 hours but if people want to let them.