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 $16.30   11.224%   $18.1295   $18.13   $18.15   $18.10  
  Purchase #2 $5.44   5.493%   $5.7388   $5.74   $5.75   $5.70  
  Purchase #3 $121.93   6.139%   $129.4153   $129.42   $129.40   $129.40  
  Purchase #4 $124.42   4.376%   $129.8646   $129.86   $129.85   $129.90  
  Purchase #5 $128.53   12.641%   $144.7775   $144.78   $144.80   $144.80  
  Purchase #6 $56.55   3.735%   $58.6621   $58.66   $58.65   $58.70  
  Purchase #7 $53.98   5.590%   $56.9975   $57.00   $57.00   $57.00  
  7 purchase totals = $507.15       $543.5853   $543.59   $543.60   $543.60