I don't know what grains you have and whether you are steeping (putting grains in just to extract flavors and sugars) or mashing (holding a temperature to accelerate an enzymatic reaction that breaks starches down into sugars). But as a general rule, make sure the temperatures never exceed 170F or you will have tannins extracted (suck on a wet tea bag for a few hours to get the flavor).
If you are actually doing a partial mash, then temperature is even more important. If you mash much lower than 145F you may not get as much enzymatic conversion of starches to sugars as would be preferred. This doesn't "ruin" beer, as you could just hold the temperatures for longer, but it might take much longer. For some recipes, you want to mash long and at low temperatures to maximize conversion to sugars (usually for thinner, lighter bodied beers).
If you mash much above 155F or so, the enzymes denature and you are stuck with lots of unfermentable malt and starches. In some recipes this is desireable as that makes the beer thicker and sweeter.
So yeah, you want to keep between 145 and 155 in general, but for some recipes you want to stay closer to 145 and for others closer to 155. Split the difference and try to hold 150, you should be OK, especially if you are using extracts.