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 $244.81   11.677%   $273.3965   $273.40   $273.40   $273.40  
  Purchase #2 $90.51   12.216%   $101.5667   $101.57   $101.55   $101.60  
  Purchase #3 $219.27   9.428%   $239.9428   $239.94   $239.95   $239.90  
  Purchase #4 $95.27   12.802%   $107.4665   $107.47   $107.45   $107.50  
  Purchase #5 $236.67   9.227%   $258.5075   $258.51   $258.50   $258.50  
  Purchase #6 $138.57   6.292%   $147.2888   $147.29   $147.30   $147.30  
  Purchase #7 $192.84   7.448%   $207.2027   $207.20   $207.20   $207.20  
  7 purchase totals = $1,217.94       $1,335.3715   $1,335.38   $1,335.35   $1,335.40