It will affect logs that you want to create road dynos from. Those are pretty sensitive.

General tuning... you do not want to do all level ground pulls. If you're inexperienced, you don't have an inherent understanding of how to tune for different loadings of the car. You have to explore those different loadings on your own. Do some uphill, do some downhill, do some in 2nd, do some in 4th... A/C on, A/C off, etc... This will give you an understanding of how different loadings change boost response. So from this standpoint, slight uphills are no issue at all. The only time it's an issue is if you're trying to benchmark road dyno levels.

Then do some tip-in tuning and 1-4 rips to verify AFR response on large throttle changes / check out for shift knock.