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 $9.30   8.356%   $10.0771   $10.08   $10.10   $10.10  
  Purchase #2 $89.90   5.122%   $94.5047   $94.50   $94.50   $94.50  
  Purchase #3 $5.45   10.661%   $6.031   $6.03   $6.05   $6.00  
  Purchase #4 $29.60   12.782%   $33.3835   $33.38   $33.40   $33.40  
  Purchase #5 $21.36   9.756%   $23.4439   $23.44   $23.45   $23.40  
  Purchase #6 $168.38   12.171%   $188.8735   $188.87   $188.85   $188.90  
  Purchase #7 $172.47   12.560%   $194.1322   $194.13   $194.15   $194.10  
  7 purchase totals = $496.46       $550.4459   $550.43   $550.50   $550.40