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 $20.85   10.347%   $23.0073   $23.01   $23.00   $23.00  
  Purchase #2 $29.97   3.691%   $31.0762   $31.08   $31.10   $31.10  
  Purchase #3 $209.57   3.334%   $216.5571   $216.56   $216.55   $216.60  
  Purchase #4 $102.26   12.547%   $115.0906   $115.09   $115.10   $115.10  
  Purchase #5 $56.79   4.721%   $59.4711   $59.47   $59.45   $59.50  
  Purchase #6 $230.45   5.167%   $242.3574   $242.36   $242.35   $242.40  
  Purchase #7 $30.27   7.513%   $32.5442   $32.54   $32.55   $32.50  
  7 purchase totals = $680.16       $720.1039   $720.11   $720.10   $720.20