Do You Sign up to Be "Kept Up-to-Date" When Entering Competitions?

I only do so when mandatory.

But I use an email address specifically for competitions for whenever I do receive any marketing emails.

If signing up to email newsletters isn't a requirement, then you are still able to win the prize. Good luck.

It's a lot harder for companies to legally force you into signing up for any form of marketing with the stricter data protection rules (GDPR). They certainly shouldn't be basing your chances on whether you signed up for marketing or not. For a company giving something away, that prize replaces the cost of them paying for advertising as it has enticed you in to look at their wares or services. It's likely to be a much lower financial risk giving away a product or service as opposed to paying a form of media for advertising. For that reason, it wouldn't be worth the risk of harming their reputation by doing anything along the lines of what you suggested.

I've always wondered the same thing,whether it hurts your chances if you don't tick the boxes at the bottom of the comps.I usually tick them and then unsubscribe when they send me an E-mail.

Yes I have always had that feeling too,if you don't show interest they may rule you out!

Only if it's required. Although, I will sign up for newsletters if they offer exclusive competitions that non-subscribers can't enter.