Certain grips make certain strokes a bit easier or feel more "natural".
This does not mean that you cannot hit heavy topspin with a continental (Laver did it) or hit flat balls with a western (Nadal does it on grass).
What is important in every case is the path of the racket head through the ball.
So, though it may take some extra condentration/focus on your part, it can be done. What you have to do is imagine you are hitting through the dead center of the ball-- straight through, like an arrow-- aiming low over the net with almost no low to high followthrough. In fact, at first, it may feel awkward-- more like you are hitting slice rather than flat-- it is a psychological thing.
Your other option is to modify your grip for the shot when you decide to hit flat, and re-grip to more of an eastern style-- (though I would not normally advise changing around to a lot of different grips).
Usually, one chooses a grip style because it suits ones requirements and style of play. Clay-courters who prefer baseline manuvering, consistancy and creating angles opt for the western.
Players who tend to have a more all-court game and play on faster surfaces go for something less than a full western.
So, the question is; what are your ultimate goals, what kind of game do you see yourself playing and on what surface?
If you are not intending to play clay courts exclusively, I would suggest a less extreme grip for all-round play.