No. I did this, flat out asked my writers "are you comfortable writing me a strong letter of recommendation?". A couple of writers did say they did not know me well enough to write a strong letter, so I sought others who could and I think it paid off for me. IMO, the only people who should be writing your a LOR are those who can write a strong one, since a weak letter can be perceived negatively, akin to "damning with faint praise".