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 $114.86   11.735%   $128.3388   $128.34   $128.35   $128.30  
  Purchase #2 $49.92   11.508%   $55.6648   $55.66   $55.65   $55.70  
  Purchase #3 $124.46   5.605%   $131.436   $131.44   $131.45   $131.40  
  Purchase #4 $125.60   10.336%   $138.582   $138.58   $138.60   $138.60  
  Purchase #5 $10.51   13.760%   $11.9562   $11.96   $11.95   $12.00  
  Purchase #6 $11.26   12.312%   $12.6463   $12.65   $12.65   $12.60  
  Purchase #7 $11.54   11.614%   $12.8803   $12.88   $12.90   $12.90  
  7 purchase totals = $448.15       $491.5044   $491.51   $491.55   $491.50