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 $27.29   13.675%   $31.0219   $31.02   $31.00   $31.00  
  Purchase #2 $230.54   8.261%   $249.5849   $249.58   $249.60   $249.60  
  Purchase #3 $85.26   12.873%   $96.2355   $96.24   $96.25   $96.20  
  Purchase #4 $122.33   9.605%   $134.0798   $134.08   $134.10   $134.10  
  Purchase #5 $61.70   7.528%   $66.3448   $66.34   $66.35   $66.30  
  Purchase #6 $27.81   13.819%   $31.6531   $31.65   $31.65   $31.70  
  Purchase #7 $243.96   8.613%   $264.9723   $264.97   $264.95   $265.00  
  7 purchase totals = $798.89       $873.8923   $873.88   $873.90   $873.90