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 $249.93   7.187%   $267.8925   $267.89   $267.90   $267.90  
  Purchase #2 $80.61   13.257%   $91.2965   $91.30   $91.30   $91.30  
  Purchase #3 $179.53   8.274%   $194.3843   $194.38   $194.40   $194.40  
  Purchase #4 $181.88   5.654%   $192.1635   $192.16   $192.15   $192.20  
  Purchase #5 $227.79   5.767%   $240.9266   $240.93   $240.95   $240.90  
  Purchase #6 $246.60   11.157%   $274.1132   $274.11   $274.10   $274.10  
  Purchase #7 $226.66   9.473%   $248.1315   $248.13   $248.15   $248.10  
  7 purchase totals = $1,393.00       $1,508.9081   $1,508.90   $1,508.95   $1,508.90