Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $12.48   5.216%   $13.131   $13.13   $13.15   $13.10  
  Purchase #2 $36.96   7.110%   $39.5879   $39.59   $39.60   $39.60  
  Purchase #3 $215.75   9.351%   $235.9248   $235.92   $235.90   $235.90  
  Purchase #4 $191.49   8.542%   $207.8471   $207.85   $207.85   $207.80  
  Purchase #5 $36.49   8.248%   $39.4997   $39.50   $39.50   $39.50  
  Purchase #6 $48.84   3.687%   $50.6407   $50.64   $50.65   $50.60  
  Purchase #7 $94.58   9.848%   $103.8942   $103.89   $103.90   $103.90  
  7 purchase totals = $636.59       $690.5254   $690.52   $690.55   $690.40