Лёха is much more informal than Алёша, I'd even say it's a bit crude (well, at least to my tender ears ). For example, it's natural for a schoolteacher to call a student Алёша but she won't call him Лёха unless she intends to be informal or humorous. But, yes, it's very common, there's even a stupid song that has the line: "Ах, Лёха, Лёха, Лёха, мне без тебя так плохо" .