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 $60.77   10.566%   $67.191   $67.19   $67.20   $67.20  
  Purchase #2 $51.94   8.222%   $56.2105   $56.21   $56.20   $56.20  
  Purchase #3 $32.86   9.689%   $36.0438   $36.04   $36.05   $36.00  
  Purchase #4 $224.68   6.273%   $238.7742   $238.77   $238.75   $238.80  
  Purchase #5 $13.75   9.476%   $15.053   $15.05   $15.05   $15.10  
  Purchase #6 $56.29   6.535%   $59.9686   $59.97   $59.95   $60.00  
  Purchase #7 $109.41   9.315%   $119.6015   $119.60   $119.60   $119.60  
  7 purchase totals = $549.70       $592.8426   $592.83   $592.80   $592.90