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 $81.56   7.637%   $87.7887   $87.79   $87.80   $87.80  
  Purchase #2 $100.39   13.508%   $113.9507   $113.95   $113.95   $114.00  
  Purchase #3 $47.65   9.206%   $52.0367   $52.04   $52.05   $52.00  
  Purchase #4 $61.36   11.629%   $68.4956   $68.50   $68.50   $68.50  
  Purchase #5 $135.95   3.667%   $140.9353   $140.94   $140.95   $140.90  
  Purchase #6 $151.99   3.555%   $157.3932   $157.39   $157.40   $157.40  
  Purchase #7 $87.70   13.518%   $99.5553   $99.56   $99.55   $99.60  
  7 purchase totals = $666.60       $720.1555   $720.17   $720.20   $720.20