You never need to think about whether there is a valid top-up on the card or not; just take the trip. You also avoid having to go to a ticket sales point or a Västtrafik shop to top up your card.
If you are not going to travel for a little while, e.g. if you are going on holiday, you can wait before activating the next top up. The card will only be topped up when you hold it against the card reader. If you have an Auto top-up period, you must activate your period within 180 days, otherwise it will become invalid.