Is it a bad practice to use an if-statement without curly braces?

The problem with the first version is that if you go back and add a second statement to the if or else clauses without remembering to add the curly braces, your code will break in unexpected and amusing ways.

Maintainability-wise, it's always smarter to use the second form.

EDIT: Ned points this out in the comments, but it's worth linking to here, too, I think. This is not just some ivory-tower hypothetical bullshit: https://www.imperialviolet.org/2014/02/22/applebug.html

One problem with leaving out statement blocks is the else-ambiguity. That is C-inspired languages ignore indentation and so have no way of separating this:

if(one)
    if(two)
        foo();
    else
        bar();

From this:

if(one)
    if(two)
        foo();
else
    bar();

My general pattern is that if it fits on one line, I'll do:

if(true) do_something();

If there's an else clause, or if the code I want to execute on true is of significant length, braces all the way:

if(true) {
    do_something_and_pass_arguments_to_it(argument1, argument2, argument3);
}

if(false) {
    do_something();
} else {
    do_something_else();
}

Ultimately, it comes down to a subjective issue of style and readability. The general programming world, however, pretty much splits into two parties (for languages that use braces): either use them all the time without exception, or use them all the time with exception. I'm part of the latter group.