Answer: It is almost always a good idea to add organic matter to the soil. Once the leaves are tilled under they will decay sooner or later and help feed the soil and improve its structure. To speed up the process, some gardeners will compost the leaves first and then add them to the garden.
Tilled in leaves would not encourage pest or disease problems unless they were infested with something that overwinters on them. Most pests are fairly host specific, so something that would say eat carrots is unlikely to also eat old tree leaves.
It is however important not to leave diseased or infested plant debris in the garden or add it to the compost or till it in since they could contribute to carryover of the problem from one year to the next. This would apply to vegetable debris in particular. For example, diseased tomato plants should be removed immediately and all remaining tomato plants should be pulled and destroyed (burning or trash, not compost) at the end of the season if there have been any problems on them.
I hope this helps.
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