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 $245.34   12.202%   $275.2764   $275.28   $275.30   $275.30  
  Purchase #2 $54.78   13.569%   $62.2131   $62.21   $62.20   $62.20  
  Purchase #3 $113.46   8.680%   $123.3083   $123.31   $123.30   $123.30  
  Purchase #4 $72.68   6.107%   $77.1186   $77.12   $77.10   $77.10  
  Purchase #5 $178.35   12.784%   $201.1503   $201.15   $201.15   $201.20  
  Purchase #6 $177.41   13.202%   $200.8317   $200.83   $200.85   $200.80  
  Purchase #7 $22.47   5.391%   $23.6814   $23.68   $23.70   $23.70  
  7 purchase totals = $864.49       $963.5798   $963.58   $963.60   $963.60