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 $19.92   4.376%   $20.7917   $20.79   $20.80   $20.80  
  Purchase #2 $15.95   13.620%   $18.1224   $18.12   $18.10   $18.10  
  Purchase #3 $199.96   8.501%   $216.9586   $216.96   $216.95   $217.00  
  Purchase #4 $178.97   5.407%   $188.6469   $188.65   $188.65   $188.60  
  Purchase #5 $74.89   7.149%   $80.2439   $80.24   $80.25   $80.20  
  Purchase #6 $144.57   7.368%   $155.2219   $155.22   $155.20   $155.20  
  Purchase #7 $142.99   7.125%   $153.178   $153.18   $153.20   $153.20  
  7 purchase totals = $777.25       $833.1634   $833.16   $833.15   $833.10