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 $18.85   5.641%   $19.9133   $19.91   $19.90   $19.90  
  Purchase #2 $82.69   9.518%   $90.5604   $90.56   $90.55   $90.60  
  Purchase #3 $127.83   12.668%   $144.0235   $144.02   $144.00   $144.00  
  Purchase #4 $20.75   12.556%   $23.3554   $23.36   $23.35   $23.40  
  Purchase #5 $247.45   10.412%   $273.2145   $273.21   $273.20   $273.20  
  Purchase #6 $121.58   13.710%   $138.2486   $138.25   $138.25   $138.20  
  Purchase #7 $186.40   9.415%   $203.9496   $203.95   $203.95   $203.90  
  7 purchase totals = $805.55       $893.2653   $893.26   $893.20   $893.20