Yes, hiding can be chosen by pressing Command-H (Or Windows-H, for PCish keyb users), or using avast!'s menu item, as :Mac: stated.

The current version will pop its GUI after startup anyway (for more reasons), but the very next one will contain dedicated hide-mode in preferences (because, for example, hiding from startup items is broken on 10.4.x etc., so in general it would be better when app controls the visibility on its own).

PC