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 $50.53   12.576%   $56.8847   $56.88   $56.90   $56.90  
  Purchase #2 $237.48   13.338%   $269.1551   $269.16   $269.15   $269.20  
  Purchase #3 $191.35   9.104%   $208.7705   $208.77   $208.75   $208.80  
  Purchase #4 $230.62   4.523%   $241.0509   $241.05   $241.05   $241.10  
  Purchase #5 $18.50   5.603%   $19.5366   $19.54   $19.55   $19.50  
  Purchase #6 $105.97   4.125%   $110.3413   $110.34   $110.35   $110.30  
  Purchase #7 $235.41   4.505%   $246.0152   $246.02   $246.00   $246.00  
  7 purchase totals = $1,069.86       $1,151.7543   $1,151.76   $1,151.75   $1,151.80