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 $8.68   13.350%   $9.8388   $9.84   $9.85   $9.80  
  Purchase #2 $12.56   3.654%   $13.0189   $13.02   $13.00   $13.00  
  Purchase #3 $117.27   9.233%   $128.0975   $128.10   $128.10   $128.10  
  Purchase #4 $81.28   7.141%   $87.0842   $87.08   $87.10   $87.10  
  Purchase #5 $43.69   10.732%   $48.3788   $48.38   $48.40   $48.40  
  Purchase #6 $80.35   3.640%   $83.2747   $83.27   $83.25   $83.30  
  Purchase #7 $213.25   8.790%   $231.9947   $231.99   $232.00   $232.00  
  7 purchase totals = $557.08       $601.6876   $601.68   $601.70   $601.70