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 $64.97   12.799%   $73.2855   $73.29   $73.30   $73.30  
  Purchase #2 $5.69   10.853%   $6.3075   $6.31   $6.30   $6.30  
  Purchase #3 $40.32   8.435%   $43.721   $43.72   $43.70   $43.70  
  Purchase #4 $59.77   5.428%   $63.0143   $63.01   $63.00   $63.00  
  Purchase #5 $179.93   10.414%   $198.6679   $198.67   $198.65   $198.70  
  Purchase #6 $154.89   8.201%   $167.5925   $167.59   $167.60   $167.60  
  Purchase #7 $42.66   12.356%   $47.9311   $47.93   $47.95   $47.90  
  7 purchase totals = $548.23       $600.5198   $600.52   $600.50   $600.50