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 $34.65   10.691%   $38.3544   $38.35   $38.35   $38.40  
  Purchase #2 $250.59   5.327%   $263.9389   $263.94   $263.95   $263.90  
  Purchase #3 $7.73   4.806%   $8.1015   $8.10   $8.10   $8.10  
  Purchase #4 $93.36   13.748%   $106.1951   $106.20   $106.20   $106.20  
  Purchase #5 $62.95   8.811%   $68.4965   $68.50   $68.50   $68.50  
  Purchase #6 $238.60   10.862%   $264.5167   $264.52   $264.50   $264.50  
  Purchase #7 $203.32   8.212%   $220.0166   $220.02   $220.00   $220.00  
  7 purchase totals = $891.20       $969.6197   $969.63   $969.60   $969.60