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 $131.77   6.261%   $140.0201   $140.02   $140.00   $140.00  
  Purchase #2 $106.26   7.629%   $114.3666   $114.37   $114.35   $114.40  
  Purchase #3 $189.58   9.184%   $206.991   $206.99   $207.00   $207.00  
  Purchase #4 $55.53   6.755%   $59.2811   $59.28   $59.30   $59.30  
  Purchase #5 $120.84   7.340%   $129.7097   $129.71   $129.70   $129.70  
  Purchase #6 $12.81   6.279%   $13.6143   $13.61   $13.60   $13.60  
  Purchase #7 $160.65   8.677%   $174.5896   $174.59   $174.60   $174.60  
  7 purchase totals = $777.44       $838.5724   $838.57   $838.55   $838.60