It's great to always have some bitcoins because they are the most stable and reliable, and you usually need bitcoins in order to buy any altcoins. So I think its a good idea to have bitcoins available when you find an alt coin you want to invest in.

Similar to stocks, it's a good idea to have a diversified portfolio. Alt coins are similar to penny stocks to me in the sense that they are much riskier but have a greater potential for profit.