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 $235.31   12.479%   $264.6743   $264.67   $264.65   $264.70  
  Purchase #2 $136.73   12.731%   $154.1371   $154.14   $154.15   $154.10  
  Purchase #3 $106.28   3.610%   $110.1167   $110.12   $110.10   $110.10  
  Purchase #4 $36.85   11.353%   $41.0336   $41.03   $41.05   $41.00  
  Purchase #5 $44.85   3.585%   $46.4579   $46.46   $46.45   $46.50  
  Purchase #6 $234.43   5.793%   $248.0105   $248.01   $248.00   $248.00  
  Purchase #7 $36.66   10.174%   $40.3898   $40.39   $40.40   $40.40  
  7 purchase totals = $831.11       $904.8199   $904.82   $904.80   $904.80