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 $39.63   5.139%   $41.6666   $41.67   $41.65   $41.70  
  Purchase #2 $55.79   10.835%   $61.8348   $61.83   $61.85   $61.80  
  Purchase #3 $219.99   9.501%   $240.8912   $240.89   $240.90   $240.90  
  Purchase #4 $124.30   10.687%   $137.5839   $137.58   $137.60   $137.60  
  Purchase #5 $67.76   10.456%   $74.845   $74.85   $74.85   $74.80  
  Purchase #6 $250.70   5.114%   $263.5208   $263.52   $263.50   $263.50  
  Purchase #7 $105.83   12.438%   $118.9931   $118.99   $119.00   $119.00  
  7 purchase totals = $864.00       $939.3354   $939.33   $939.35   $939.30