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 $28.47   8.392%   $30.8592   $30.86   $30.85   $30.90  
  Purchase #2 $109.72   9.842%   $120.5186   $120.52   $120.50   $120.50  
  Purchase #3 $12.31   5.141%   $12.9429   $12.94   $12.95   $12.90  
  Purchase #4 $174.34   4.401%   $182.0127   $182.01   $182.00   $182.00  
  Purchase #5 $7.86   5.343%   $8.28   $8.28   $8.30   $8.30  
  Purchase #6 $136.91   11.264%   $152.3315   $152.33   $152.35   $152.30  
  Purchase #7 $80.61   7.541%   $86.6888   $86.69   $86.70   $86.70  
  7 purchase totals = $550.22       $593.6337   $593.63   $593.65   $593.60