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 $21.84   12.230%   $24.511   $24.51   $24.50   $24.50  
  Purchase #2 $96.84   6.286%   $102.9274   $102.93   $102.95   $102.90  
  Purchase #3 $14.81   11.861%   $16.5666   $16.57   $16.55   $16.60  
  Purchase #4 $53.89   12.374%   $60.5583   $60.56   $60.55   $60.60  
  Purchase #5 $225.75   8.501%   $244.941   $244.94   $244.95   $244.90  
  Purchase #6 $186.60   8.676%   $202.7894   $202.79   $202.80   $202.80  
  Purchase #7 $149.88   4.255%   $156.2574   $156.26   $156.25   $156.30  
  7 purchase totals = $749.61       $808.5511   $808.56   $808.55   $808.60