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 $12.32   7.415%   $13.2335   $13.23   $13.25   $13.20  
  Purchase #2 $119.47   7.592%   $128.5402   $128.54   $128.55   $128.50  
  Purchase #3 $33.77   5.290%   $35.5564   $35.56   $35.55   $35.60  
  Purchase #4 $63.53   7.602%   $68.3596   $68.36   $68.35   $68.40  
  Purchase #5 $170.93   11.315%   $190.2707   $190.27   $190.25   $190.30  
  Purchase #6 $177.28   8.536%   $192.4126   $192.41   $192.40   $192.40  
  Purchase #7 $18.44   3.150%   $19.0209   $19.02   $19.00   $19.00  
  7 purchase totals = $595.74       $647.3939   $647.39   $647.35   $647.40