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 $121.43   5.462%   $128.0625   $128.06   $128.05   $128.10  
  Purchase #2 $76.44   5.646%   $80.7558   $80.76   $80.75   $80.80  
  Purchase #3 $129.60   9.634%   $142.0857   $142.09   $142.10   $142.10  
  Purchase #4 $15.67   6.318%   $16.66   $16.66   $16.65   $16.70  
  Purchase #5 $133.98   11.266%   $149.0742   $149.07   $149.05   $149.10  
  Purchase #6 $85.85   5.176%   $90.2936   $90.29   $90.30   $90.30  
  Purchase #7 $68.56   4.729%   $71.8022   $71.80   $71.80   $71.80  
  7 purchase totals = $631.53       $678.7340   $678.73   $678.70   $678.90