From my experience on a selections committee, teaching (labs, tutoring, teaching HS for a few years, etc.) was seen as a positive -- since most accepted graduate students start their first term or first year with guaranteed financial support in the form of a TA (teaching assistant-ship) rather than an RA (research assistant-ship -- for which they must talk to faculty and arrange AFTER being accepted to the program, and most students spend their first year taking core coursework and talking to faculty on the side about possible research projects).
However, while the effect of teaching experience on acceptance is existent, it is VERY, VERY slight. As Andy mentions above, a Ph.D. program is generally designed to train the candidate for research (not teaching) This is true even though the majority of "academic" jobs are at schools where undergraduate education is significant. Note that competition for any academic job is tough, and it's a greater likelihood that you'll land a non-academic research position, either at a company or at a national lab, etc.
Generally the place to put any teaching experience on the graduate schools application would be on your resume/CV (if the school's application calls for one) or as a very small part in your personal statement and your professors' supporting recommendations (though these latter two pieces of your application should mostly concentrate on your RESEARCH EXPERIENCE).