CPUs have set tech patterns and are not actually very good options at all. You start to pick up on them (whether you realize or not) and real people don't tech in quite the same ways. PM makes it hard because of the lack of 20xx AI that actually mixes up options. Better is to actively try to techchase in friendlies and progressively improve on it. Theorycrafting is also an essential function because it allows you to see specifically which situations and actions are the most difficult for you to cover, and thus they change how you would want to position yourself. Theorycrafting in the realm of the techchase is all about positioning yourself optimally for a followup: that would mean being able to cover every option on reaction with little difficulty.
Common knowledge states that the tech in place is the fastest recovery option an opponent has (as it has the least frame commitment and the highest percentage of intangibility), so you want to put yourself close enough to punish the tech in place: right next to where your opponent lands. The next thing to consider is the tech rolls: they tend to be roughly the same distance both forward and backward, the clincher is that if the opponent has to roll through your character model as opposed to away from it, then that distance becomes shorter for you (because they spend some amount of time getting "closer" to you. The final thing to consider—and a lot of people forget about this—is the no-tech options. You can jab reset these, but it's hard to react to if you're constantly thinking "they're going to tech." This is why I tend to favor wavedashing to position myself as opposed to dashing, since I can jab much more easily than doing turnarounds or dash-cancels (or you can just use attack stick and not really need to worry*). Players can actually utilize the no-techs as a valuable mixup tool, considering as well that they then get four NEW options to utilize if you fail to cover the no-tech, and these don't have a set instant in which they need to happen (like how techs MUST happen when they touch ground). The same options still apply as with the techs, but you're absolutely required to have good reactions to punish them, and you also have to deal with the get-up attack—an option not really present in tech situations. I'm not positive, but I feel like Sheik is a character whose jab can be SDI'd up to avoid being jab reset, so that's also something to be wary of and is usually combated with Jab 2.
*Setting C-Stick to "Attack" rather than "Smash" makes a c-stick forward input an f-tilt and a c-stick back input a jab. Combine this with dash-cancelling (holding down while in a dash) allows for frame-perfect dash-stop jabs. I use it a lot with Snake and when I do play Sheik I tend to keep this control scheme the same because jab and f-tilt >> f-smash.
I would imagine (although I'm not positive) that the tech roll away on Sheik's down throw (assuming DI away) is the most difficult to cover, and I would also assume logically most people are aware of this. Therefore, I would try to dash into a waveland shield pivot or turnaround to get behind the opponent as a way to cover the most likely options (i.e. tech in place and tech roll away). Sheiks wavedash is super fast and most characters' tech rolls aren't that long. So I'd try that out and see how it works. If it doesn't, I might just maintain the basic structure and try to cover the roll away with a boost grab. It all depends on the character as well, because different characters actually have different roll lengths which are easier or harder to cover, as well as varying amounts of invincibility on those rolls.
You also need to consider the techchase within the context of the stage: Sheik's downthrow puts them at a good distance away from you, and DI in is asking to get comboed, so it's a good positioning tool that eventually forces them into a corner. Depending on your opponent, they may feel comfortable or uncomfortable in the corner, and their tech actions will reflect this to some degree (although conscious players will mixup their techs regardless of positioning).
a tl;dr for this accidentally massive post: play against real people, and be conscious about what is happening at all stages of the game.