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 $245.97   12.617%   $277.004   $277.00   $277.00   $277.00  
  Purchase #2 $23.66   12.205%   $26.5477   $26.55   $26.55   $26.50  
  Purchase #3 $159.59   7.446%   $171.4731   $171.47   $171.45   $171.50  
  Purchase #4 $145.34   9.640%   $159.3508   $159.35   $159.35   $159.40  
  Purchase #5 $44.28   4.345%   $46.204   $46.20   $46.20   $46.20  
  Purchase #6 $101.99   12.304%   $114.5388   $114.54   $114.55   $114.50  
  Purchase #7 $220.61   6.530%   $235.0158   $235.02   $235.00   $235.00  
  7 purchase totals = $941.44       $1,030.1342   $1,030.13   $1,030.10   $1,030.10