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 $134.37   3.405%   $138.9453   $138.95   $138.95   $138.90  
  Purchase #2 $85.94   6.812%   $91.7942   $91.79   $91.80   $91.80  
  Purchase #3 $179.85   4.352%   $187.6771   $187.68   $187.70   $187.70  
  Purchase #4 $207.59   13.310%   $235.2202   $235.22   $235.20   $235.20  
  Purchase #5 $116.55   4.742%   $122.0768   $122.08   $122.10   $122.10  
  Purchase #6 $115.92   6.234%   $123.1465   $123.15   $123.15   $123.10  
  Purchase #7 $79.55   12.208%   $89.2615   $89.26   $89.25   $89.30  
  7 purchase totals = $919.77       $988.1216   $988.13   $988.15   $988.10