What is difference between 1 and 1. you may see in these examples:
In[100]:= 1 + 3/5
Out[100]= 8/5
And next try with 1.
In[99]:= 1. + 3/5
Out[99]= 1.6
As you see in first example Mathematica try to give EXACT result because you don't use decimal point. When any number in an arithmetic expression is given with an explicit decimal point - as in your case 1. - you get an APPROXIMATE numerical result for the whole expression.
These APPROXIMATIONS in complex equations affect the output result just because they are approximations not exact numbers. End Result much depends what time of the complex calculations you serve your REQUEST FOR APPROXIMATION to Mathematica with some variable as "K1 = 1.;"
This topic is more like academic discussion than a simple answer.
P.s. In your concrete case I don't see any difference when I try it!
If the result is different obviously it matters whether you define in advance any of the variables as decimal point number or not. Like these two example.
In first case if you give whole number argument of f[] result will be exact.
Clear["Global`*"]
a = 2;
b = 5;
f[c_] := (a*b)/c;
In[172]:= f[3]
Out[172]= 10/3
In second case result will be approximation because a is 2.
Clear["Global`*"]
a = 2.;
b = 5;
f[c_] := (a*b)/c;
In[191]:= f[3]
Out[191]= 3.33333
but if use instead 3 argument 3. no matter what variable a is (2 or 2.) result will be approximation in the two cases.
Clear["Global`*"]
a = 2;
b = 5;
f[c_] := (a*b)/c;
In[209]:= f[3.]
Out[209]= 3.33333
and
Clear["Global`*"]
a = 2.;
b = 5;
f[c_] := (a*b)/c;
In[223]:= f[3.]
Out[223]= 3.33333
Do you understand?
In your case difference can be only if you use variables Kpi, Kii, R, L as exact numbers
For examples this result:
T1 = 180*10^-6;
K1 = 1;
sysCL[Kpi_, Kii_, R_, L_] := K1/(T1 s + 1)*(Kpi + Kii/s)*1/(R + L s)
BodePlot[sysCL[3, 427, 2, 2*10^-3]]
will be different from this result:
T1 = 180*10^-6;
K1 = 1.;
sysCL[Kpi_, Kii_, R_, L_] := K1/(T1 s + 1)*(Kpi + Kii/s)*1/(R + L s)
BodePlot[sysCL[3, 427, 2, 2*10^-3]]