I'll usually use the 0.30" for tacking and where I have good solid steel which will take the heat. For those curved areas where the metal has been "stress-formed" or "stretched", I switch over to 0.25".
Good quality wire is a must. Lincoln brand is all I will use. Learned THAT lesson the hard way.
http://store.lincolnelectric.com/webapp/wcs/stores/servlet/en/leesite/consumables/mild-steel/mig-wire/superarc-reg%3B-l-56-reg%3B-superarcl-56
Gas flow can be adjusted also which can help a bit.
Yeah, welding those flat or smoothly curved pieces they use in schools is a lot different then when you're sitting inside of a trunk, laying under the car or being a contortionist trying to reach that spot which is just behind the clamp which is holding your part in alignment.