Rental cosighn

I'm thinkink about co-sighning for a rental agreeement for a family member which isn't the issue because I have the income to make sure that the rent is covered, however my concern is whether or not with being a co sighner will I still be able to buy vehicles or a home,etc? I've heard as long as you can prove the income that you have the income ( including the co-sighned rental) this is still possible.Does any one know if this is true