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 $127.60   8.689%   $138.6872   $138.69   $138.70   $138.70  
  Purchase #2 $113.73   13.613%   $129.2121   $129.21   $129.20   $129.20  
  Purchase #3 $93.96   7.323%   $100.8407   $100.84   $100.85   $100.80  
  Purchase #4 $15.38   3.873%   $15.9757   $15.98   $16.00   $16.00  
  Purchase #5 $33.76   7.788%   $36.3892   $36.39   $36.40   $36.40  
  Purchase #6 $16.26   9.640%   $17.8275   $17.83   $17.85   $17.80  
  Purchase #7 $10.89   11.395%   $12.1309   $12.13   $12.15   $12.10  
  7 purchase totals = $411.58       $451.0633   $451.07   $451.15   $451.00