After
You sould have the new tires mounted FIRST, then have them do the alignment at the same visit.
Before-a bad alignment cannot "unbalance" a wheel, but an unbalanced wheel, if bad enough, can throw off an alignment.
Good idea to do at the same time
Yes.
My van has bad tires at front and better ones at rear, should wheel alignment be performed with bad tires?
Tires do not get out of alignment they may be out of balance but not alignment. Your suspension however can be out of alignment. The only way to know for sure is to have it checked.
If your tires are balanced and it's still pulling, then your car is ot of alignment. Go to a local tire shop to have an alignment done and that should rectify the problem.
Not necessarily. If the car was in alignment prior to getting the new tires, it will be in alignment after the new tires. It isn't a bad idea to check the alignment periodically and it is probably better to check it more frequently than when you get new tires.
If you changed them yourself it is recommended that you get an alignment done or do the alignment yourself. If a shop or mechanic did the work they should have done the alignment themselves.
new tires does not mean you don't need alignment. it is always a good idea to have the vehicle aligned after new tires are purchased to avoid premature wear, even if the car shows no sign of mis-alignment.
There is no need to have a cars wheels aligned on a regular bases. It is a good idea though to have the alignment checked when installing new tires to prevent excess wear on the tires.
Your alignment could be off. Meaning your tires are either facing out a bit, or facing in a bit. This can cause your tires to go bad. Your local garage should be able to do a alignment on your vehicle to straighten things up.