From my experience banks will crunch numbers and come back with what they believe you can afford at the time. "AT THE TIME" being key here. This does not mean that you should go with that. It just gives you an idea of what the bank feels your buying power is. As others have mentioned you will want to take into consideration the "what ifs" but there will always be "what ifs." Don't let it be a stopper in what you want. Just keep it in mind as you move forward and plan accordingly. Many things will be a bit vague and blurry with the numbers but the numbers from the banks are usually on the high end on the monthly mortgage for the reason that you can shop for some on your own and get them for less (home insurance being one).
Also note that locking in an interest rate should NOT cost you anything. A bank will generally lock in the interest rate upon the initial completion and acceptance of the pre-approval process. If a bank wants to charge to lock you in, look elsewhere. Lot's of lenders. Go with the one that you feel most comfortable with and listens. Interest rate should be locked in for a certain period of time (90, 120, 180 days, etc) at which rate it'll restart at the current.
Buying property is both exciting and stressful. Stress can be negated some if you have all your finances figured before hand. It can be stressful trying to find the ideal place which you never will. Keep in mind that you can and will make the place yours. Color is cosmetic so if you find a house you like but the rooms are painted purple with orange dots look beyond what it is and look at what it could be. Layout is very important unless you are handy enough or have the means to hire to remove/add walls, etc. Go into the rooms and imagine how you would use it. Walk the paths. You are generally in a house for a short time when looking and are expected to make a decision about living in it for possibly a long time. Spend the time and slow down when looking around. Spend the most time walking around and looking in the areas you use the most (generally kitchen, toilet and bathroom) and those that are important to each other. Also, pay attention to what is outside the place. Are there apartments going up across the street or is a new freeway going to be built behind your place. Always recall the realtors motto "Location, location location" when looking.
Size of a place is also important. You don't want overly large beyond your usage and you don't want so small you are wishing you had a gone larger. Go as large as you are comfortable with. Having a spare room is great. More storage is fantastic. Having to have guests or an office in the living room not fun. Having to cram everything into a closet or two is not good. Get the space while you can.
One thing I have done in the years of buying and selling is I ask myself if this is the place I want to stay or is it going to be a stepping stone to the place I want to be. If a stepping stone go into with the mindset that it is an investment and I want the most money in my pocket and not to the bank. If not then you make it what you really want and not what some else may want and possibly consider a shorter mortgage. You can always refinance at a later time if things change.
Overall, I say enjoy the ride and experience. Sorry for running off topic a bit.