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 $27.55   12.238%   $30.9216   $30.92   $30.90   $30.90  
  Purchase #2 $133.80   7.684%   $144.0812   $144.08   $144.10   $144.10  
  Purchase #3 $192.43   12.257%   $216.0161   $216.02   $216.00   $216.00  
  Purchase #4 $36.44   7.842%   $39.2976   $39.30   $39.30   $39.30  
  Purchase #5 $221.61   8.379%   $240.1787   $240.18   $240.20   $240.20  
  Purchase #6 $142.55   13.290%   $161.4949   $161.49   $161.50   $161.50  
  Purchase #7 $15.46   5.855%   $16.3652   $16.37   $16.35   $16.40  
  7 purchase totals = $769.84       $848.3553   $848.36   $848.35   $848.40