Parameters of Variant calling analysis
We will be needing some additional information to answer your question more completely:

What sequencing platform are you using? Illumina HiSeq/MySeq; Ion proton

What type of sequencing (as noted by @CMosychuk): Exome or Whole Genome?

What type of variant caller (as noted by @CMosychuk): Unified Genotyper; Haplotype Caller; TVC

The QUAL metric is heavily dictated my sequencing depth, so if you use 30 (phred-scaled), that would be an accuracy of 99.9%.

The typical QUAL metric used by most sequencing studies is 20 so using 30 may cause accumulation of false negatives. But again, depends on the type of sequencing and platform used.