I have begun to reuse liquid yeast from my homebrewing and was wondering whether it is better to save yeast from the primary fermentation or the secondary fermentation? I've heard that it is a decision whether to save "dirtier but vigourous yeast cells", or "cleaner but tired yeast cells".

G'day Drs, About while ago I was bottling a batch of hefeweizen and wanted to reserve some yeast from it. My method was to rinse about 100mL thick yeast slurry in cooled boiled water once or twice, removing as much trub as possible, before leaving it under a 300mL layer of cooled boiled water and storing in the fridge at 1C. The strain was White Labs Hefeweizen (WLP300).

Can you please discuss the advantages/disadvantages of pitching onto an existing yeast cake (assuming the cake, and previous beer, smelled and tasted clean).