I think 5 btc is a huge amount to invest in altcoins and if you are new to trading then its not recommended to go with that high amount, its always good to start with a small capital and gain experience first so I think if you are going to trade altcoins then 0.5 btc is a good amount to start with as even if you loose it won't make much difference to you.
In my experience, even 0.1 bitcoin is more than enough to consider as a decent investment for altcoin trading. But it is highly dependent on experience of a trader, after gaining experience no single trader is preferring to go with minimal amounts.
Calculated risk is the biggest advantage of experienced traders, so they can go with any amount of investments but only after gaining handsome experience with trading.
0.1 btc is a decent investment for me as well and use some of it for trading.. as a starter in trading it's better to invest with a small capital first. Once you already know how the system works in trading that' s time you can invest with a big capital.