Is range validity checked for when performing range operations on GenomicRanges::GRanges ?

Yes, it is.

Example GenomicRanges:

require(magrittr)
bsgenome <- BSgenome.Mmusculus.UCSC.mm10::BSgenome.Mmusculus.UCSC.mm10
granges <- GenomicRanges::GRanges(seqnames = 'chr1', 
                                  ranges   = IRanges::IRanges(start = 100, end = 200), 
                                  strand   = '-', 
                                  seqinfo  = BSgenome::seqinfo(bsgenome))

Coordinates < 1 warn:

GenomicRanges::start(granges) %<>% subtract(500)

Warning message:
In valid.GenomicRanges.seqinfo(x, suggest.trim = TRUE) :
GRanges object contains 1 out-of-bound range located on sequence chr1. Note that ranges located on a sequence whose length is unknown (NA) or on a circular sequence are not considered out-of-bound (use seqlengths() and isCircular() to get the lengths and  circularity flags of the underlying sequences). You can use trim() to trim these ranges. See   ?`trim,GenomicRanges-method` for more information.

coordinates > chrlength also do:

GenomicRanges::end(granges) %<>% add(1 + GenomeInfoDb::seqlengths(bsgenome)[['chr1']])

Warning message:
In valid.GenomicRanges.seqinfo(x, suggest.trim = TRUE) :
GRanges object contains 1 out-of-bound range located on sequence chr1. Note that ranges located on a sequence whose length is unknown (NA) or on a circular sequence are not considered out-of-bound (use seqlengths() and isCircular() to get the lengths and  circularity flags of the underlying sequences). You can use trim() to trim these ranges. See   ?`trim,GenomicRanges-method` for more information.