The thing is that the human naked eye is not as sensitive as plants are to the strength/power of light, so even if your lighting looks as bright as it did 10-12 months ago the light that is being emmited will be lower than when the bulb was new.
This will make a difference to your plants, it will not be that noticable in a low tech set up but will be more notcable in a high tech set up. WIth my LED tiles I can set them at 85% and then anything higher up to 100% is not noticable to my eyes. After swapping my T5 bulbs after 12 months use I would notice my plants pearling a lot heavier than they did with the 12 month old tubes.
Like Mike said using bulbs till they blow is fine, your plants wont die off they will just adapt to the lower levels of light and grow slower.