I think much would depend on what fishes are being kept.
I don't think daily three or four degree temp change would be too bad but my expierience has indicated that fishes like corydoras and more than a few tetra species actually do better at lower temps with possible exception of cardinal's and perhaps rummy nose.
Can only speak from my own expierience's but most of the tera's I keep,
Flame,neon's,glowlight's,serpae,blackskirt's,seem to thrive and live much longer in temps around 75 degree's F. When kept above 78 degree's for extended period they only seem to last but a few month's perhaps a year.
Same thing with corydoras. I'm near certain that there are plenty who keep tetra's and cory's in much warmer water(to each his/her own), but I have much better luck long term with cooler water. Even my plant's seem to do better at cooler temps .Recent AC malfunction (capacitor) resulted in two day's of near 90 degree water in my tanks, and fishes and plant's did not take it very well despite twice daily frozen litres of water placed in the tanks and water changes with cooler water.(lost around a dozen neon's)
Depending on the light purchased,,this could raise the surface temp in your tank a bit more and as mentioned,depending on species of fishes kept,may or may not have impact.
I have not found many fishes that did not appreciate temps somewhere in the middle of suggested ranges as opposed to upper or lower extremes and I have much better luck with longevity of the fishes I keep than I used to in year's past.
If your tanks water does not get below 65 to 70 degree's F in the cooler months,you may not need a heater unless keeping fishes like Discus or some of the dwarf cichlid's .IMHO
