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 $17.70   5.610%   $18.693   $18.69   $18.70   $18.70  
  Purchase #2 $90.50   3.666%   $93.8177   $93.82   $93.80   $93.80  
  Purchase #3 $131.94   10.618%   $145.9494   $145.95   $145.95   $145.90  
  Purchase #4 $116.28   12.368%   $130.6615   $130.66   $130.65   $130.70  
  Purchase #5 $149.59   11.170%   $166.2992   $166.30   $166.30   $166.30  
  Purchase #6 $145.69   13.695%   $165.6422   $165.64   $165.65   $165.60  
  Purchase #7 $135.76   6.202%   $144.1798   $144.18   $144.20   $144.20  
  7 purchase totals = $787.46       $865.2428   $865.24   $865.25   $865.20