Google and other major search engines don't discourage A/B testing or personalization. Read more about it in Website testing and Google search.
Google does its utmost to avoid inadvertently penalizing search rankings of sites that employ testing and optimization. To this end, it provides several key pointers to ensure your testing remains as effective as possible and ultimately provides the best user experience.
No Cloaking
Cloaking—showing one set of content to humans and a different set to Googlebot—is against Google's Webmaster Guidelines, whether you're running a test or not. Ensure that you're not deciding whether to serve the test or which content variant to serve based on user-agent.
Always serving the original content when you see the user-agent Googlebot is an example. Remember that infringing Google's guidelines can get your site demoted or removed from Google search results—probably not the desired outcome of your test.
Use rel="canonical"
If you run an A/B test with multiple URLs, you can use the rel="canonical" link attribute on all your alternate URLs to indicate that the original URL is the preferred version. Monetate recommends using rel="canonical" rather than a noindex metatag because it more closely matches your intent in this situation.
If you're testing variations of your homepage, you don't want search engines to avoid indexing your homepage. You want them to understand that all the test URLs are close duplicates or variations on the original URL and should be grouped as such with the original URL as the canonical. Using noindex rather than rel="canonical" can sometimes have unexpected effects.
Consider the following example. If for some reason you choose one of the variant URLs as the canonical, the "original" URL might also get dropped from the index since it would get treated as a duplicate.
Use 302s, not 301s
If you run an A/B test that redirects customers from the original URL to a variation URL, use a 302 (temporary) redirect not a 301 (permanent) redirect. This tells search engines that this redirect is temporary—it only remains in place as long as you're running the experiment—and that they should keep the original URL in their index rather than replacing it with the target of the redirect (the test page). JavaScript-based redirects are also fine.
Run Your Experiment Only as Long as Necessary
The amount of time required for a reliable test varies depending on factors, such as conversion rates and how much traffic your site gets. A good testing tool should tell you when you've gathered enough data to draw a reliable conclusion.
Once you've concluded the test, update your site with the desired content variation(s) and remove all elements of the test as soon as possible. These testing elements may include alternate URLs or testing scripts and markup. If Google discovers a site running an experiment for an unnecessarily long time, it may interpret this as an attempt to deceive search engines and take action accordingly. This is especially true if you serve one content variant to a large percentage of your users.