V/I uppercase/lowercase convention?
I agree with you that it is important to know the meaning of the different ways for using such symbols. And the same applies also to the voltage-to-current ratios (resitances, impedances). For my opinion the standard is (or should be) as follows:


Uppercase (V,I) for DC and rms values 
Uppercase for ohmic resistors R=V/I
Lowercase (v,i) for signals as a function of time: v(t), i(t)  
Lowercase (v,i) for (small) differential signals available for a certain DC bias point only.
Lowercase (r) for differential (dynamic) small-signal resistances r=v/i.


As a negative example, in small-signal equivalent circuits, sometimes the inverse transconductance gm of a BJT is used as Re=1/gm. This is very confusing because this does not represent any (ohmic) resistor and, more than that, can be mixed-up with an external emitter resistor RE.      

EDIT/UPDATE: 
 Regarding impedances:

For reactive elements (L, C) the voltage-to-current ratios are called "impedance". Because this applies to rms values of sinusoidal signals only the symbol for impedances also is written in uppercase letters Z=V/I.