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 $135.54   5.595%   $143.1235   $143.12   $143.10   $143.10  
  Purchase #2 $96.67   3.178%   $99.7422   $99.74   $99.75   $99.70  
  Purchase #3 $163.45   7.574%   $175.8297   $175.83   $175.85   $175.80  
  Purchase #4 $31.56   10.577%   $34.8981   $34.90   $34.90   $34.90  
  Purchase #5 $87.78   10.477%   $96.9767   $96.98   $97.00   $97.00  
  Purchase #6 $59.58   4.821%   $62.4524   $62.45   $62.45   $62.50  
  Purchase #7 $24.39   12.337%   $27.399   $27.40   $27.40   $27.40  
  7 purchase totals = $598.97       $640.4216   $640.42   $640.45   $640.40