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 $29.40   8.439%   $31.8811   $31.88   $31.90   $31.90  
  Purchase #2 $128.33   4.792%   $134.4796   $134.48   $134.50   $134.50  
  Purchase #3 $7.37   9.142%   $8.0438   $8.04   $8.05   $8.00  
  Purchase #4 $114.82   12.404%   $129.0623   $129.06   $129.05   $129.10  
  Purchase #5 $100.90   6.788%   $107.7491   $107.75   $107.75   $107.70  
  Purchase #6 $139.40   3.641%   $144.4756   $144.48   $144.50   $144.50  
  Purchase #7 $12.63   5.355%   $13.3063   $13.31   $13.30   $13.30  
  7 purchase totals = $532.85       $568.9978   $569.00   $569.05   $569.00