From the master himself

I know it's a long time ago, but in an interview with Bill Venners, on November 17, 2003, the master Yukihiro Matsumoto himself said:

So for day-to-day programs that aren't not as serious as enterprise systems, you don't have to be as robust. It's not worth the cost of declaring types, for example.

So he agrees: declaring types has a cost, but it adds robustness.

http://www.artima.com/intv/tuesday3.html