OK, let's do it.

koj よいことをするのにためらうな、の意。
daijs よいことはためらわずすぐに行え。
nikk: よいことをするのに躊躇(ちゅうちょ)するな、善事はただちに実行せよの意。
gakkoku: よいことだと思ったらためらわずに急いで実行せよ。
ことわざを知る辞典: よいことはためらわず、ただちに実行せよ。

I'm not 100% sure よいことをするのに should be interpreted as about doing something at an opportune moment/when the timing is right, but if you look at nikk's def for 悪は延べよ: "悪いと思う事は、一事延期せよ、そうすれば事情がやがて変化して、行なわないで済むようになることもある。⇔善は急げ", then, maybe?

@@ -14,0 +15 @@
+<gloss>strike while the iron is hot</gloss>

I came to this entry to add "strike while the iron is hot" as a
gloss, but I see it was once added and removed. Can we
reconsider? The definitions given in daij, oukoku, and nikkoku
are all something similar to 「よいことは機会を逃さず急いでせよ」,
which to me sounds like a perfect match for the English idiom.

Yes, it's a pun on the proverb, and it means something like "serving food quickly". Definitely not to go in this entry.
With 1181 in the n-grams, I feel it should get an entry somewhere. I'll dig a bit more.

@@ -6,4 +5,0 @@
-</k_ele>
-<k_ele>
-<keb>膳は急げ</keb>
-<ke_inf>&iK;</ke_inf>

If it's pun on the proverb, I don't think 
it belongs in this entry.
(I also don't think it's common enough to 
have its own entry, unless it's 
covered/mentioned by any of the kokugo's)