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 $17.87   11.699%   $19.9606   $19.96   $19.95   $20.00  
  Purchase #2 $9.43   11.113%   $10.478   $10.48   $10.50   $10.50  
  Purchase #3 $168.52   9.297%   $184.1873   $184.19   $184.20   $184.20  
  Purchase #4 $21.72   11.523%   $24.2228   $24.22   $24.20   $24.20  
  Purchase #5 $219.37   9.378%   $239.9425   $239.94   $239.95   $239.90  
  Purchase #6 $243.39   6.343%   $258.8282   $258.83   $258.85   $258.80  
  Purchase #7 $39.83   4.783%   $41.7351   $41.74   $41.75   $41.70  
  7 purchase totals = $720.13       $779.3545   $779.36   $779.40   $779.30