Usually when BTC rises, the alts rise more, and as a rule, when BTC fall, the alts devaluate too when compared to BTC,
If you think prices will rise, then you should keep some alts just in case and do less diversification to keep it manageable, if you think prices will fall, the best is sell everything to BTC then sell the BTC and wait
Nah, unless you are an aggressive day/short trader with inside information that's pointless really. Just keep your BTC and you'll see you won't gain much in that period holding other altcoins, it's just not worthy. Plus keep in mind the amount of time and stress it involves.