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 $56.63   4.163%   $58.9875   $58.99   $59.00   $59.00  
  Purchase #2 $140.79   10.548%   $155.6405   $155.64   $155.65   $155.60  
  Purchase #3 $26.27   12.177%   $29.4689   $29.47   $29.45   $29.50  
  Purchase #4 $238.30   11.203%   $264.9967   $265.00   $265.00   $265.00  
  Purchase #5 $111.90   8.155%   $121.0254   $121.03   $121.05   $121.00  
  Purchase #6 $206.49   13.226%   $233.8004   $233.80   $233.80   $233.80  
  Purchase #7 $58.89   13.529%   $66.8572   $66.86   $66.85   $66.90  
  7 purchase totals = $839.27       $930.7766   $930.79   $930.80   $930.80