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 $29.34   5.590%   $30.9801   $30.98   $31.00   $31.00  
  Purchase #2 $79.98   11.222%   $88.9554   $88.96   $88.95   $89.00  
  Purchase #3 $9.60   3.528%   $9.9387   $9.94   $9.95   $9.90  
  Purchase #4 $155.96   10.773%   $172.7616   $172.76   $172.75   $172.80  
  Purchase #5 $35.73   6.728%   $38.1339   $38.13   $38.15   $38.10  
  Purchase #6 $12.63   10.486%   $13.9544   $13.95   $13.95   $14.00  
  Purchase #7 $67.88   10.855%   $75.2484   $75.25   $75.25   $75.20  
  7 purchase totals = $391.12       $429.9725   $429.97   $430.00   $430.00