Lately, I've seen some users (who shall remain nameless) upvote answers by new users simply as a 'traditional welcoming gift'. An upvote is not the traditional welcoming gift (it's usually a nice comment). If a post by a new user is good and worthy, by all means, upvote it. If it's a so-so post or even just wrong, don't go sympathy upvoting to 'welcome' the user. Help him/her out by leaving a constructive comment. Vote for the post, not the user.
Sympathy voting has two major problems: it encourages so-so to just wrong posts and it can remove all new user restrictions for that user.
When a new user writes a not-so-great post and it's upvoted out of sympathy (rather than because it was a great post), the new user now has a distorted view of what is a quality post and what needs work. Leave a constructive comment to help them a along, but don't upvote the post because it's a first post.
New user restrictions are removed at 10 rep. Sympathy upvoting an answer removes all of these restrictions, but the user has not really earned them. New user restrictions are precautionary measures. Sympathy upvoting destroys the whole point of them.
So just keep in mind that the next time you see the first post of a new user, you're shaping their SE experience. Don't hesitate to leave a constructive comment. It goes a long way in helping the user get used to SE and how it works. However, don't go sympathy voting if the vote is undeserved. A constructive comment goes a long way farther than a careless upvote.
Read When should I vote? for more information on when to vote.