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 $217.95   12.873%   $246.0067   $246.01   $246.00   $246.00  
  Purchase #2 $149.49   7.367%   $160.5029   $160.50   $160.50   $160.50  
  Purchase #3 $5.88   8.178%   $6.3609   $6.36   $6.35   $6.40  
  Purchase #4 $171.48   4.352%   $178.9428   $178.94   $178.95   $178.90  
  Purchase #5 $27.78   3.627%   $28.7876   $28.79   $28.80   $28.80  
  Purchase #6 $96.72   3.193%   $99.8083   $99.81   $99.80   $99.80  
  Purchase #7 $95.98   8.295%   $103.9415   $103.94   $103.95   $103.90  
  7 purchase totals = $765.28       $824.3507   $824.35   $824.35   $824.30