Whether it's safe very much depends on what you count as „safe“. All that might resulting from giving LeadBolt your IMEI is that you won't be shown any ads by LeadBolt – but if you think that means „no ads at all“, you're completely mistaken. There are tons of other ad networks used by apps.¹ Some of them offer you the same way to „opt out“ by giving them your IMEI. Nevertheless, they'll continue collecting data on you, connecting it to your IMEI – which you now even explicitly give them (probably combined with some other data), to build a profile on you and maybe even sell it for money.
I'm currently writing an article on that topic (see below). My advice would be: Don't. That's the wrong way to act on ads. You should not „register“ your IMEI (or any other identifier) with a company to have it stop annoying you. Rather use other approaches to get rid of those annoying ads:
- contact the developer of that app and tell him/her about the annoyance, asking to replace this ad provider by one less intrusive
- if the app has a paid version that comes without ads², buy and use that. After all, the dev put hours or even months of work into the app, which should be honored
- to find out what ad modules an app contains, check it out at AppBrain. Avoid apps which contain intrusive ad modules (compare e.g. with my list¹).
- use an Ad Blocker. That's nothing dishonest in this context, it's pure self-protection considering several of those ad networks also ship „malvertising“ stuff, and what PII they collect on you without your consent or awareness. On Android, you don't need a so called „Anti-Virus“ solution, you need a good working „Anti-Ad“ protection.
For further reading I recommend you my article Android Identifiers: How Android devices and their users are identified. Hopefully the article I've mentioned above will be available at the same site at the end of this months, to give deeper insight on What´s it all about those modules apps contain? – i.e. ad and analytics stuff and how it intrudes our privacy.
¹ for some especially intrusive examples, see my listing here
² believe it or not, but about every seventh paid app ships with ad modules