You don't have to have absolutely perfect IVs but you will be at a tiny disadvantage at the Nationals or Worlds level since your opponents will primarily have Pokemon with perfect IVs. R Inanimate made Top 16 at the US Nationals without RNG'd Pokemon but even he still had Pokemon with an average IV of 28 or more (for relevant stats so this excludes things like SpAtk IV on a Pokemon without special attacks) and IV31 in important stats. If you don't RNG, you also pretty much have to give up on getting usable roaming legendaries like Tornadus or Thundurus (especially Hidden Power Thundurus). Also Hidden Power is a lot less accessible without RNG. These are problems you can get around by making the right kind of team, though. You can without a doubt still be very successful using Pokemon you bred yourself.