A starter will definitely help as it will give you a lot more yeast. (and prove their viability) Most likely you'll be fine. Someone will eventually link the "Fermentation can take up to 72 hours to show visible signs" thread. In the future I'd look at making a starter. The simplest way is use 1/2 cup dme to 2 cups water. Boil it for 5-15 minutes, cool it, and pitch the vial into that. A mason jar works pretty well for holding the starter. Aerate it by shaking it up whenever you pass by it. Let it go for 24-72 hours at 65-70 degrees F to let it multiply a few bajillion times. Then you can just pitch the whole mason jar of yeast/beer into your 5 gallon batch. You should see a massive improvement in your lag time.
You can also get all fancy with starters. You can make different gravity starters. You can make a bunch of starter ahead of time and pressure-can it. You can buy/build a magnetic stir plate and get a 2L glass flask and a stir bar to constantly aerate the starter. You could get an O2 tank and a diffusion stone to oxygenate the starter. You can step it up from 500mL to 5L and get a ton of yeast. You can then save some and slant it or freeze it with glycerin. It gets crazy quickly. The first paragraph has the easiest method.
All-in-all, you'll probably be fine this time around. Wait 72 hours before adding yeast. If all else fails, get a packet of dry yeast and throw it in after the HBT-mandated 72 hr waiting time.
