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 $219.94   12.817%   $248.1297   $248.13   $248.15   $248.10  
  Purchase #2 $23.69   11.671%   $26.4549   $26.45   $26.45   $26.50  
  Purchase #3 $37.94   8.635%   $41.2161   $41.22   $41.20   $41.20  
  Purchase #4 $72.91   8.385%   $79.0235   $79.02   $79.00   $79.00  
  Purchase #5 $206.58   8.675%   $224.5008   $224.50   $224.50   $224.50  
  Purchase #6 $38.65   9.626%   $42.3704   $42.37   $42.35   $42.40  
  Purchase #7 $237.48   3.560%   $245.9343   $245.93   $245.95   $245.90  
  7 purchase totals = $837.19       $907.6297   $907.62   $907.60   $907.60