For those prospective breeders out there who also want to dabble in training Legendaries for competitive battle, a word to the wise about them: you’re only guarenteed to get halfway-decent ones from those that are stationary. Stationary Legendaries (Mewtwo, Lugia, Virizion, etc) are easy to soft-reset catch to roll for random IVs because all you have to do is save in front of it, talk to it, catch it, check the IVs, and soft reset reroll if you don’t like what you’ve gotten, and repeat. Using that method, you’re pretty well guarenteed to catch a usable Legendary Pokemon. Roaming Legendaries are different. Those that do roam (their location changes in every instance you enter, such as Mesprit, Latios/Latias, Entei and Raikou) have their IVs and nature fixed from the moment of “encounter”, that is, when they start to roam. No matter where you are, where you’ve saved, or how many times you soft reset and try again, their nature and IVs will not change from point of encounter forward.
That being said, it’s very likely that you will encounter the need to fill your Dex pages and need them, or you’ll want to use them for competitive battle. Well, despair not. In terms of Latios and Latias, they can both be found and caught by stationary encounter in HeartGold and SoulSilver versions, respectively. That’s about the end of the good news. As for Entei and Raikou, the two begin to roam very early in HeartGold and SoulSilver, and you’ll likely not be in any sort of position to catch or even battle evenly with the two Legendaries. The only way to get reliably randomized IV Entei and Raikou is through the event promotion released shortly before the sale of Black and White. Same with Mesprit. Mesprit, however, is somewhat more manageable, seeing as it only starts to roam once you meet it in its cave after the events corresponding with Dialga (Diamond), Palkia (Pearl), or the Distortion World (Platinum). If you save right before speaking to it, you could in theory have the opportunity to let it roam, find it, catch it, check its IVs, and if it is dissatisfactory, restart from before you spoke to it. However, like the rather impractical capture of Entei and Raikou, from a competitive sense, catching a usable Mesprit is equally draining in terms of time and patience invested. It’s almost better to let the three become placeholders in your Pokedex rather than become actual competitive Pokemon.