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 $24.90   11.600%   $27.7884   $27.79   $27.80   $27.80  
  Purchase #2 $11.73   5.689%   $12.3973   $12.40   $12.40   $12.40  
  Purchase #3 $116.88   3.562%   $121.0433   $121.04   $121.05   $121.00  
  Purchase #4 $31.51   10.537%   $34.8302   $34.83   $34.85   $34.80  
  Purchase #5 $171.77   7.412%   $184.5016   $184.50   $184.50   $184.50  
  Purchase #6 $217.55   5.405%   $229.3086   $229.31   $229.30   $229.30  
  Purchase #7 $13.32   4.558%   $13.9271   $13.93   $13.95   $13.90  
  7 purchase totals = $587.66       $623.7965   $623.80   $623.85   $623.70