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 $28.58   6.568%   $30.4571   $30.46   $30.45   $30.50  
  Purchase #2 $204.26   12.836%   $230.4788   $230.48   $230.50   $230.50  
  Purchase #3 $93.32   7.296%   $100.1286   $100.13   $100.15   $100.10  
  Purchase #4 $25.80   6.338%   $27.4352   $27.44   $27.45   $27.40  
  Purchase #5 $57.33   7.276%   $61.5013   $61.50   $61.50   $61.50  
  Purchase #6 $145.52   11.322%   $161.9958   $162.00   $162.00   $162.00  
  Purchase #7 $28.27   7.273%   $30.3261   $30.33   $30.35   $30.30  
  7 purchase totals = $583.08       $642.3229   $642.34   $642.40   $642.30