One thing to check for. Every hard hat I have ever used has a warning label on it that says not to apply any adhesive that is not approved by the manufacturer. I know that stickers are applied to hard hats every day (even some given out by Safety and Health), but if a sticker is applied and it doesn't use an adhesive approved by the manufacturer then the hard hat no longer meets ANSI standards because the adhesive could have affected the plastic.
Like I said I know it is done every day at every facility, and the application of one sticker to a hard hat will not change its effectiveness, but the manufacturer put that warning there for a reason.