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 $5.27   9.694%   $5.7809   $5.78   $5.80   $5.80  
  Purchase #2 $13.62   11.211%   $15.1469   $15.15   $15.15   $15.10  
  Purchase #3 $146.82   13.397%   $166.4895   $166.49   $166.50   $166.50  
  Purchase #4 $228.51   9.311%   $249.7866   $249.79   $249.80   $249.80  
  Purchase #5 $43.40   4.145%   $45.1989   $45.20   $45.20   $45.20  
  Purchase #6 $24.71   12.264%   $27.7404   $27.74   $27.75   $27.70  
  Purchase #7 $8.67   12.759%   $9.7762   $9.78   $9.80   $9.80  
  7 purchase totals = $471.00       $519.9194   $519.93   $520.00   $519.90