There are several things that I consider: First is the type of restoration or repair I'm doing; if it's a collectible TV with historic value, I will go to extremes to get a new cap with sufficiently long leads and will remove the old leads and make it like the factory did. If it's a quick repair on something that doesn't have much historic value (predictas, etc, where it's a HUGE job to do it right,) I'll clip the leads. If there's an IF transformer, or other thing that's going to be easy to damage by heat, I may make a judgement call and clip the lead going to that part, and make a loop with the remaining old lead, and solder the new cap to the loop.
In general, if I'm recapping a 630 or something, I will most always do it right and remove the old leads. Just words of wisdom: Chances are that some day, another collector will have their hands on the TV that you're recapping. Think about what people would say when they look at your work
The Late Roy Burnett, for example, was known for his beautiful work on restoring Scott and McMurdo Silver radios. Every lead from every component was laid perfectly. Every corner square and straight, every wire stripped to the exact same length, every resistor with exactly 1.5 turns of it's leads through the lug, every terminal strip plumb and true.
Charles