The simple answer is no. Research has shown that there is no increased risk from lifting with a bent back. Relaxed, free movement is best for your back rather than holding it stiff and tense. Bending over to tie your shoes or weed the flower bed should be a natural free movement which will involve bending your back.
The only time where it may be advisable to avoid excessive bending of the back whilst lifting is when lifting heavy loads. It is probably advisable to try to maintain a neutral position in these situations.