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 $248.82   13.483%   $282.3684   $282.37   $282.35   $282.40  
  Purchase #2 $99.27   9.651%   $108.8505   $108.85   $108.85   $108.90  
  Purchase #3 $250.95   8.571%   $272.4589   $272.46   $272.45   $272.50  
  Purchase #4 $163.96   5.851%   $173.5533   $173.55   $173.55   $173.60  
  Purchase #5 $5.80   4.243%   $6.0461   $6.05   $6.05   $6.00  
  Purchase #6 $148.75   8.203%   $160.952   $160.95   $160.95   $161.00  
  Purchase #7 $17.50   7.317%   $18.7805   $18.78   $18.80   $18.80  
  7 purchase totals = $935.05       $1,023.0097   $1,023.01   $1,023.00   $1,023.20