water current is good for ur plants & will keep them healthy!! with no or very low current algae seems to grow over them and consuming them
as far as how strong it should b, enough to keep the plants moving somewhat. I think it more depends on how big a tank and how heavily planted.
I have a 75 gal thats heavily planted with a
canister filter and i have so many plants in it that i keep a powerjet in there too, otherwise the current doesnt make the travel of the length of the tank because its offset from the plants. I have pics in my profile of that tank its pretty dense