Honestly I wouldn't bother, the Bismarck is the best ship in that line and going further for me at least resulted in a lot of frustration. I stopped at the tier 9 and wished I hadn't of bothered as its a lot of credits and there are better ships at that tier imho. You do need to be fairly skilled to do well in the higher tier German BBs and you see a lot of them played poorly. I'm saving my free XP for other ships, sure the tier 10 looks good and I've also heard good things but personally there are better places for the free XP to go imho. I bring out the tier 9 on 200% days and such to progress it slowly but its rarely enjoyable, some of that is to do with my skill level, some of that is to do with the MM and ships you go up against.
For me at least the biggest problem with the German battleships at tier 9 and 10 is they are brawlers but without support when you get into secondary range more than likely your team mates in random will just leave you to get focused down. This is less of an issue at the lower tiers because the players tend to be less knowledgeable or at least less coordinated. They also might be hard to citadel but they take a ton of AP damage regardless.
So if you have free XP for days sure, but personally I'd put it elsewhere.