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 $51.83   8.182%   $56.0707   $56.07   $56.05   $56.10  
  Purchase #2 $232.96   8.646%   $253.1017   $253.10   $253.10   $253.10  
  Purchase #3 $60.31   13.739%   $68.596   $68.60   $68.60   $68.60  
  Purchase #4 $103.72   4.391%   $108.2743   $108.27   $108.25   $108.30  
  Purchase #5 $65.97   6.819%   $70.4685   $70.47   $70.45   $70.50  
  Purchase #6 $132.32   9.620%   $145.0492   $145.05   $145.05   $145.00  
  Purchase #7 $147.98   4.321%   $154.3742   $154.37   $154.35   $154.40  
  7 purchase totals = $795.09       $855.9346   $855.93   $855.85   $856.00