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 $201.39   10.731%   $223.0012   $223.00   $223.00   $223.00  
  Purchase #2 $177.91   6.293%   $189.1059   $189.11   $189.10   $189.10  
  Purchase #3 $24.35   6.564%   $25.9483   $25.95   $25.95   $25.90  
  Purchase #4 $43.69   10.699%   $48.3644   $48.36   $48.35   $48.40  
  Purchase #5 $248.78   8.635%   $270.2622   $270.26   $270.25   $270.30  
  Purchase #6 $6.61   3.498%   $6.8412   $6.84   $6.85   $6.80  
  Purchase #7 $230.90   6.198%   $245.2112   $245.21   $245.20   $245.20  
  7 purchase totals = $933.63       $1,008.7344   $1,008.73   $1,008.70   $1,008.70