Ok, so tell us what stance do you call a "good" one? In zenkutsu, fudo, kokutsu, hangetsu and whatever stance we practice in traditional shotokan karate, your front leg is a good target to grab. And as soon as grappler seize it, he will press his head to your body and you will have no chance to use strong karate technics just be cause there will be no space for it. Other tricks like eyes squeezing and throat crashing are not so simple to use as many of you think it is. IMHO the only thing a striker can do against a grappler is to "cause as much damage as possible , in as short a time as possible" as it's already said here. Atack groin, knees, use uppercut when he closes the distance to grab you (the don't care much about their head protection when they move to grab your leg be cause thier main idea is to close the distance at whatever cost - at least in BJJ). Hiza geri (knee kick) and uppercut are very good strikes, be cause grappler bands his body and his face becomes unprotected. Use tai sabaki actions and atack his kidney, groin and knees again and again. But never let him grab your leg. Remember that they improve thier grappling and wrestling skills at every trainig. It's useless trying to defeat them in the area, where they are Gods. But even if you find your self mounted on the ground, don't panic, use their own methods to escape from the mounted position and use use use STRIKES, bite whatever you can bite, squeeze thier balls but don't let them wrestle with you. He will catch you on the painful hold and break your arm or leg if you don't disable him before.