You really don't have to worry about tuning your guitar too much. It won't break the strings--they're stretchier than they look. Plus, they need to be changed every so often, anyway. You gotta play a guitar pretty hard to bust a string unless they're super old or you tune them way too high or something. I tune every time I play and I don't change my strings as often as I should, and yet I think I've broken maybe one or two strings in the last 15 years or so. lol

But is it bad practice? That depends. If you're playing with a band, yes. You gotta have your instrument in tune. If you're playing by yourself for fun? No, just do whatever sounds good to you; but, if you're just learning guitar, I would very much recommend keeping it in tune when you play so that you can train your ears effectively.