Do I need a blowoff tube?

I know it's a good idea to have one, but my first two batches proceeded without problems. My third batch, a summer ale I dreamed up myself to in line with Sam Summer per my wife's request, has a much more active fermentation.

It's a five-gallon batch in a 6-gallon plastic primary. I'm thinking the extra head space will mitigate the need for a blowoff tube but I'd like more expert opinions.

The recipe:
.5 lbs Cara-Pils
.25 lbs. German Munich
5 lbs. XLDME
.5 lbs. wheat DME
1 oz Cascade 6% (60 mins)
1 tsp. Irish Moss
.5 oz cascade (15 mins)
1 oz dried lemon peel (5 mins)
.5 tsp. seeds of paradise
.5 oz Cascade (flameout)
Wyeast 1332 Northwest Ale (no starter)

Thanks again for the help.

You will eventually need a blow off with a 6 gallon bucket. I exploded my 6.5 gallon once, and have even plugged up a 7.9 gallon fermenter once on a 5.5 gallon batch. Better safe than sorry.

Blowoff tube has been fashioned in quick order. There was a ton of pressure building on the primary lid, so thanks to you all I figured it was a good idea to do just in case.