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 $123.64   3.750%   $128.2765   $128.28   $128.30   $128.30  
  Purchase #2 $145.27   13.498%   $164.8785   $164.88   $164.90   $164.90  
  Purchase #3 $230.72   13.729%   $262.3955   $262.40   $262.40   $262.40  
  Purchase #4 $99.57   3.540%   $103.0948   $103.09   $103.10   $103.10  
  Purchase #5 $198.60   4.220%   $206.9809   $206.98   $207.00   $207.00  
  Purchase #6 $175.89   7.471%   $189.0307   $189.03   $189.05   $189.00  
  Purchase #7 $216.57   3.781%   $224.7585   $224.76   $224.75   $224.80  
  7 purchase totals = $1,190.26       $1,279.4154   $1,279.42   $1,279.50   $1,279.50