That's a tough question to answer because everyone is going to be different. As a general rule, I'd only ask someone if they need assistance if you can see they appear lost or confused by their location. Someone who can't get around without company (cross roads etc) is unlikely to be doing so on their own.

At the end of the day though, it never hurts to ask, just make sure you are obvious when you approach.

Study our body language. If we look a little lost or confused, approach and ask if we need any help, plus identifying yourself always helps. Do not grab us without consent, especially if we are attempting to line up for a street crossing. If help is requested, ask us what we would like you to do so we can give you a very quick rundown of proper orientation and mobility tactics to safely move around. If attempting to give directions, try using Clock directions where 12 o'clock is directly in front of the person who is visually impaired that you are addressing. Don't be persistent if we say no, nothing is more frustrating than trying to shoo away an overly excited good Samaritan when we are in Full control of our orientation and situation.

Apply the same rules as with any other stranger. If they are obviously about to hurt themselves or are visibly struggling with something, ask if they need a hand. Personally I'm very aware of my abilities, and I've likely planned around anything I know will be difficult for me. If I've exhausted all my tools and workarounds, I'll ask for help.

You can always ask if someone needs help. I've had people try to grab my arm and it scares me.

for me its a matter of seeing crosswalk signs in the bright light all you have to do is allow me to walk with you crossing the street with the green light and not j walk