Is it OK to use Math.Pow (10, n)?

There are way more performant implementations of integer Pow, than multiplying by 10 in a loop.

See this answer, for example, it also works for other base values, not just 10.

Also, consider caching results, up until a reasonable amount, in an array.

Here's my recent code, for a reference:

private static readonly Int64[] PowBase10Cache = {
    1,
    10,
    100,
    1000,
    10000,
    100000,
    1000000,
    10000000,
    100000000,
    1000000000,
    10000000000,
    100000000000,
    1000000000000,
    10000000000000,
    100000000000000,
    1000000000000000,
    10000000000000000,
    100000000000000000,
    1000000000000000000,
    };

public static Int64 IPowBase10(int exp)
{
    return exp < PowBase10Cache.Length ? PowBase10Cache[exp] : IPow(10L, exp);
}

public static Int64 IPow(Int64 baseVal, int exp)
{
    Int64 result = 1;
    while (exp > 0)
    {
        if ((exp & 1) != 0)
        {
            result *= baseVal;
        }
        exp >>= 1;
        baseVal *= baseVal;
    }
    return result;
}