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 $16.85   7.469%   $18.1085   $18.11   $18.10   $18.10  
  Purchase #2 $21.85   6.456%   $23.2606   $23.26   $23.25   $23.30  
  Purchase #3 $165.35   4.291%   $172.4452   $172.45   $172.45   $172.40  
  Purchase #4 $39.42   11.159%   $43.8189   $43.82   $43.80   $43.80  
  Purchase #5 $155.49   3.235%   $160.5201   $160.52   $160.50   $160.50  
  Purchase #6 $11.37   12.508%   $12.7922   $12.79   $12.80   $12.80  
  Purchase #7 $66.80   3.461%   $69.1119   $69.11   $69.10   $69.10  
  7 purchase totals = $477.13       $500.0574   $500.06   $500.00   $500.00