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 $18.55   13.737%   $21.0982   $21.10   $21.10   $21.10  
  Purchase #2 $120.90   12.124%   $135.5579   $135.56   $135.55   $135.60  
  Purchase #3 $91.88   12.245%   $103.1307   $103.13   $103.15   $103.10  
  Purchase #4 $213.35   8.576%   $231.6469   $231.65   $231.65   $231.60  
  Purchase #5 $39.47   9.747%   $43.3171   $43.32   $43.30   $43.30  
  Purchase #6 $56.80   6.408%   $60.4397   $60.44   $60.45   $60.40  
  Purchase #7 $56.49   11.345%   $62.8988   $62.90   $62.90   $62.90  
  7 purchase totals = $597.44       $658.0893   $658.10   $658.10   $658.00