yes g-reg, the upthrow on marth and the upthrow on shiek are probably 2 of the mose useful combos against those respective matchups. there is SO MUCH LESS PRESSURE against shiek when i know that all i have to do is 80% then grab, especially since shiek is so hard to kill over 100%.
one thing i have as a suggestion to all, against marth. to get him to the 70% or so when you can upthrow knee him, if you downthrow and SHFFL a nair in to him, you can do some cool things. depending on his DI, sometimes you can even regrag. if that works, and you can shffl another nair in to him, you can double up air (the second one is the second jump off the stage). this is my zero to death against marth. it doesnt work all that often, but just doing half the combo and punishing his tech is extremely effective.
one of the biggest benefits of this is it puts him really close to the inescapable grab knee %.
shiek is harder to get to the 80% mark that seals her doom, but i find that treating her like a space animal until about 30% works pretty well. once at about 30%, i find that up throw to nair is an awesome combo on shiek. if she DI's up, then you can put ANOTHER nair in to her. if he continues to DI up, you can often put a low % knee in to her, and commence ledge guarding. after about 50%, i suggest up throw to up air, and trying to stay below her from there, as shiek does not do great when above you.
also, if you are having problems with shiek, fox, or falco (pretty much the characters have give falcon buttloads of trouble) then you can try silent spectre's drop zone. practice in training mode a bit. put them at 50%, walk them RIGHT to the edge of FD, grab them, downthrow them off. you can walk off the stage and knee. if you did it perfectly, you can double jump upB back on to the ledge. a knee this low and so far out guarentees death against these falcon counters. getting off a kill like that can REALLY throw the match in your favor.
oh and i've found spamming nairs works absolute wonders against ganondorf and link particularly, as the are fairly easy to chain one nair in to another.