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 $7.59   4.415%   $7.9251   $7.93   $7.95   $7.90  
  Purchase #2 $18.34   9.624%   $20.105   $20.11   $20.10   $20.10  
  Purchase #3 $12.82   13.869%   $14.598   $14.60   $14.60   $14.60  
  Purchase #4 $61.99   6.676%   $66.1285   $66.13   $66.15   $66.10  
  Purchase #5 $240.79   7.537%   $258.9383   $258.94   $258.95   $258.90  
  Purchase #6 $99.74   8.454%   $108.172   $108.17   $108.15   $108.20  
  Purchase #7 $30.71   11.825%   $34.3415   $34.34   $34.35   $34.30  
  7 purchase totals = $471.98       $510.2084   $510.22   $510.25   $510.10