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 $234.95   13.444%   $266.5367   $266.54   $266.55   $266.50  
  Purchase #2 $108.30   4.871%   $113.5753   $113.58   $113.60   $113.60  
  Purchase #3 $170.61   3.753%   $177.013   $177.01   $177.00   $177.00  
  Purchase #4 $38.29   3.226%   $39.5252   $39.53   $39.55   $39.50  
  Purchase #5 $32.58   4.205%   $33.95   $33.95   $33.95   $34.00  
  Purchase #6 $114.79   9.408%   $125.5894   $125.59   $125.60   $125.60  
  Purchase #7 $239.47   3.178%   $247.0804   $247.08   $247.10   $247.10  
  7 purchase totals = $938.99       $1,003.2700   $1,003.28   $1,003.35   $1,003.30