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 $68.28   13.432%   $77.4514   $77.45   $77.45   $77.50  
  Purchase #2 $115.34   5.540%   $121.7298   $121.73   $121.75   $121.70  
  Purchase #3 $169.90   6.152%   $180.3522   $180.35   $180.35   $180.40  
  Purchase #4 $68.34   6.702%   $72.9201   $72.92   $72.90   $72.90  
  Purchase #5 $8.42   4.810%   $8.825   $8.83   $8.85   $8.80  
  Purchase #6 $239.76   8.208%   $259.4395   $259.44   $259.45   $259.40  
  Purchase #7 $50.86   4.201%   $52.9966   $53.00   $53.00   $53.00  
  7 purchase totals = $720.90       $773.7146   $773.72   $773.75   $773.70