I'm not the best when it comes to sidestepping, I'm trying to learn it more and more every opportunity but it's tough -- so take what I have to say with a grain of salt. It's also pretty tough to explain because it's situational depending on what move you're blocking/ hit with.
Generally speaking, I side step after a blocked attack that doesn't visibly put yoshi at a disadvantage. Like say, generic df+1's and such. I think noodlehead stated that yoshi needs to be at -5 or better to successfully sidestep... maybe it was -4? Usually I will only attempt to step after a blocked attack, never after being hit (that's asking to get hit again). If you feel the need to step after being hit, use b33_b44. You'll take damage, but you have a better chance of escaping and if they're string happy, you may have time to get a free df+2,2_cd1 launcher.
Knowing which direction to step helps as well, and again knowing which direction to step is something that just comes with experience. Generally speaking (again), if I know what attack the opponent is going to be doing, I step towards the attacking limb. So for example, say you're playing Drag and he loves to spam that pesky while running 2 (the running overhand punch), Drag attacks with his right hand, therefore I step towards it to yoshis left (drags right). Some moves are better to side walk as well, I'm still experimenting with that (and after a quick search via Inca's yoshi bible, it seems this move can be SW'd to either side, but if you're going to step, you can only SSL).
Also, don't forget about flash. If you find yourself in a situation where you need to SS but don't have enough frames, flash might be a good option to interrupt the next attack if the opponent is close enough. Going back to drag, picture his b+1+2 (hammerfist) at the wall. Drag players like to use this because it keeps them at +frames and sets you up to possibly eat a ch 1, 2, 1 for a wallspat and big damage. Yoshi can step in between the hammerfist and punches, but he can also flash and interrupt to relieve the pressure.
I'd say to check out the yoshi bible if you're having problems with specific moves, it's helped me a lot. http://www.tekkenzaibatsu.com/forums/showthread.php?postid=3995314
Many characters tend to have a weakside as well (as in, they have tracking issues to that side), so what I did is make a "Step List" with all the characters on my phone, so I can check it real quick and know which direction is generally a good idea.
This post was edited by Junon (2012-03-20 20:25, 5 years ago)