Of course, in a modern glass cockpit - 139 for example, you get to see Nr, Nf and all other engine parameters on one screen so it is not a case of having to concentrate on the Nr gauge since all the info you need is in one place, unlike older analogue displays where manufacturers put the Nr gauge in all sorts of random locations.
The other point is that aural cues are usually available - either from an Aural Warning Generator or a simple beeping horn.
On the 365 N3 for example you knew that you had the Nr within limits when both the high and low Nr horns were silent and the low Nr horn was particularly useful for OEI work - pull until it goes off, then back off enough to quieten it again and you didn't have to look at the Nr gauge to know you had the desired 350 Nr.
I presume the 135 has similar warnings.