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 $248.64   10.409%   $274.5209   $274.52   $274.50   $274.50  
  Purchase #2 $114.72   4.800%   $120.2266   $120.23   $120.25   $120.20  
  Purchase #3 $49.95   9.263%   $54.5769   $54.58   $54.60   $54.60  
  Purchase #4 $163.51   7.420%   $175.6424   $175.64   $175.65   $175.60  
  Purchase #5 $115.93   8.582%   $125.8791   $125.88   $125.90   $125.90  
  Purchase #6 $46.85   8.529%   $50.8458   $50.85   $50.85   $50.80  
  Purchase #7 $89.27   8.775%   $97.1034   $97.10   $97.10   $97.10  
  7 purchase totals = $828.87       $898.7951   $898.80   $898.85   $898.70