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 $122.31   5.431%   $128.9527   $128.95   $128.95   $129.00  
  Purchase #2 $71.77   11.826%   $80.2575   $80.26   $80.25   $80.30  
  Purchase #3 $55.97   3.324%   $57.8304   $57.83   $57.85   $57.80  
  Purchase #4 $142.27   3.770%   $147.6336   $147.63   $147.65   $147.60  
  Purchase #5 $17.91   12.802%   $20.2028   $20.20   $20.20   $20.20  
  Purchase #6 $24.39   4.244%   $25.4251   $25.43   $25.45   $25.40  
  Purchase #7 $177.66   5.868%   $188.0851   $188.09   $188.10   $188.10  
  7 purchase totals = $612.28       $648.3872   $648.39   $648.45   $648.40