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 $160.69   7.138%   $172.1601   $172.16   $172.15   $172.20  
  Purchase #2 $35.67   11.425%   $39.7453   $39.75   $39.75   $39.70  
  Purchase #3 $36.50   6.519%   $38.8794   $38.88   $38.90   $38.90  
  Purchase #4 $160.39   13.230%   $181.6096   $181.61   $181.60   $181.60  
  Purchase #5 $155.82   7.313%   $167.2151   $167.22   $167.20   $167.20  
  Purchase #6 $27.53   13.180%   $31.1585   $31.16   $31.15   $31.20  
  Purchase #7 $8.37   8.702%   $9.0984   $9.10   $9.10   $9.10  
  7 purchase totals = $584.97       $639.8664   $639.88   $639.85   $639.90