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 $138.36   5.125%   $145.451   $145.45   $145.45   $145.50  
  Purchase #2 $227.70   7.614%   $245.0371   $245.04   $245.05   $245.00  
  Purchase #3 $108.54   5.294%   $114.2861   $114.29   $114.30   $114.30  
  Purchase #4 $40.32   4.520%   $42.1425   $42.14   $42.15   $42.10  
  Purchase #5 $43.85   7.842%   $47.2887   $47.29   $47.30   $47.30  
  Purchase #6 $10.84   7.646%   $11.6688   $11.67   $11.65   $11.70  
  Purchase #7 $30.85   3.452%   $31.9149   $31.91   $31.90   $31.90  
  7 purchase totals = $600.46       $637.7891   $637.79   $637.80   $637.80