I think so long as you aren't asking for anything out of the ordinary & you're applying pretty far in advance, you're okay with booking the ticket. I personally am waiting, because my situation is little outside the guidlines (I want to enter the UK over 3 weeks before my uni starts...the guidlines say approx 2) & there's a small risk I won't get the date I am requesting. However, for spousal visas, you don't have that issue. As long as your plans for travel aren't too soon, I think you're pretty safe.