If I put 18" tires on my 89 Jag do I have to get a front end alingment?

 

In most cases, no. However, if the current tires show ANY weird wear pattern, then definitely align it. No sense to wear out brand new tires when you needn't.
Another thing to consider is the current tread WIDTH. Some wider tires need a slightly different adjustment than do skinny ones. So if your current tire width is somewhat comparable to the new ones AND your current tires have no abnormal wear, you should be fine as it is.