You will fingd that a decent shirt and tie will travel better than in a suitcase and a lot depends on what you are joining. I wore a Jacket and tie in 1981 on the way to IJLB and I had to wear it for two days as they didnt issue us with kit till then. Jeans would have been more comfortable but they were a no no then!
If the packing list states smart civilian suit then by wearing it you are saving having to carry it and looking smart never hurt. Just remember to polish your shoes!
There you go, no mention of denim cutoffs and rambo knife!
When i did selection i saw a bunch of new guys in training getting a beasting for turning up like they were going to hang out on a street corner, served as a good reminder for when i arrived at lichfield