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 $16.90   6.472%   $17.9938   $17.99   $18.00   $18.00  
  Purchase #2 $34.86   9.197%   $38.0661   $38.07   $38.05   $38.10  
  Purchase #3 $230.65   12.198%   $258.7847   $258.78   $258.80   $258.80  
  Purchase #4 $51.66   8.234%   $55.9137   $55.91   $55.90   $55.90  
  Purchase #5 $208.82   5.676%   $220.6726   $220.67   $220.65   $220.70  
  Purchase #6 $11.51   4.867%   $12.0702   $12.07   $12.05   $12.10  
  Purchase #7 $10.55   7.386%   $11.3292   $11.33   $11.35   $11.30  
  7 purchase totals = $564.95       $614.8303   $614.82   $614.80   $614.90