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 $28.35   9.811%   $31.1314   $31.13   $31.15   $31.10  
  Purchase #2 $15.86   3.584%   $16.4284   $16.43   $16.45   $16.40  
  Purchase #3 $145.61   6.189%   $154.6218   $154.62   $154.60   $154.60  
  Purchase #4 $103.27   7.405%   $110.9171   $110.92   $110.90   $110.90  
  Purchase #5 $43.31   13.619%   $49.2084   $49.21   $49.20   $49.20  
  Purchase #6 $183.51   3.507%   $189.9457   $189.95   $189.95   $189.90  
  Purchase #7 $128.71   10.362%   $142.0469   $142.05   $142.05   $142.00  
  7 purchase totals = $648.62       $694.2997   $694.31   $694.30   $694.10