This is my opinion on the situation. I think investing a small amount is better than nothing.
Yeah this is what I've been believing all the time, it's better to have small investment rather than to have nothing at all.
You are allowed to learn about how the investment process and how it works while earning some income.
Yes, it's a matter of testing that investment on how it is going and if you'll see the potential profit upon investing bigger amount then just add it up.
If you were to gamble, you might be a winner the first day but you won't make a steady profit and have a bigger risk of losing.
Gambling isn't an investment, so I don't consider whether big or small amount you'll start with it.
Also SafeDice allows you to invest any amount in the house bankroll so you can invest 1 satoshi.
It's a literal small amount though.