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 $7.30   10.126%   $8.0392   $8.04   $8.05   $8.00  
  Purchase #2 $35.60   8.512%   $38.6303   $38.63   $38.65   $38.60  
  Purchase #3 $51.47   6.729%   $54.9334   $54.93   $54.95   $54.90  
  Purchase #4 $162.54   3.492%   $168.2159   $168.22   $168.20   $168.20  
  Purchase #5 $12.50   13.131%   $14.1414   $14.14   $14.15   $14.10  
  Purchase #6 $113.42   3.545%   $117.4407   $117.44   $117.45   $117.40  
  Purchase #7 $16.82   13.155%   $19.0327   $19.03   $19.05   $19.00  
  7 purchase totals = $399.65       $420.4336   $420.43   $420.50   $420.20