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 $247.32   13.608%   $280.9753   $280.98   $281.00   $281.00  
  Purchase #2 $178.50   13.589%   $202.7564   $202.76   $202.75   $202.80  
  Purchase #3 $91.94   12.257%   $103.2091   $103.21   $103.20   $103.20  
  Purchase #4 $161.86   9.111%   $176.6071   $176.61   $176.60   $176.60  
  Purchase #5 $77.93   11.479%   $86.8756   $86.88   $86.90   $86.90  
  Purchase #6 $82.43   8.592%   $89.5124   $89.51   $89.50   $89.50  
  Purchase #7 $218.70   11.283%   $243.3759   $243.38   $243.40   $243.40  
  7 purchase totals = $1,058.68       $1,183.3118   $1,183.33   $1,183.35   $1,183.40