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 $242.58   5.357%   $255.575   $255.58   $255.60   $255.60  
  Purchase #2 $15.68   3.312%   $16.1993   $16.20   $16.20   $16.20  
  Purchase #3 $27.33   11.707%   $30.5295   $30.53   $30.55   $30.50  
  Purchase #4 $250.96   13.180%   $284.0365   $284.04   $284.05   $284.00  
  Purchase #5 $36.52   10.768%   $40.4525   $40.45   $40.45   $40.50  
  Purchase #6 $165.92   3.182%   $171.1996   $171.20   $171.20   $171.20  
  Purchase #7 $55.37   10.287%   $61.0659   $61.07   $61.05   $61.10  
  7 purchase totals = $794.36       $859.0583   $859.07   $859.10   $859.10