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 $249.63   13.352%   $282.9606   $282.96   $282.95   $283.00  
  Purchase #2 $23.48   4.793%   $24.6054   $24.61   $24.60   $24.60  
  Purchase #3 $58.43   13.758%   $66.4688   $66.47   $66.45   $66.50  
  Purchase #4 $127.68   7.418%   $137.1513   $137.15   $137.15   $137.20  
  Purchase #5 $46.37   13.839%   $52.7871   $52.79   $52.80   $52.80  
  Purchase #6 $78.25   3.617%   $81.0803   $81.08   $81.10   $81.10  
  Purchase #7 $165.39   8.168%   $178.8991   $178.90   $178.90   $178.90  
  7 purchase totals = $749.23       $823.9526   $823.96   $823.95   $824.10