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 $19.47   7.842%   $20.9968   $21.00   $21.00   $21.00  
  Purchase #2 $13.64   10.554%   $15.0796   $15.08   $15.10   $15.10  
  Purchase #3 $125.60   13.279%   $142.2784   $142.28   $142.30   $142.30  
  Purchase #4 $102.64   6.438%   $109.248   $109.25   $109.25   $109.20  
  Purchase #5 $199.57   3.545%   $206.6448   $206.64   $206.65   $206.60  
  Purchase #6 $241.64   12.461%   $271.7508   $271.75   $271.75   $271.80  
  Purchase #7 $203.54   6.277%   $216.3162   $216.32   $216.30   $216.30  
  7 purchase totals = $906.10       $982.3146   $982.32   $982.35   $982.30