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 $28.28   5.344%   $29.7913   $29.79   $29.80   $29.80  
  Purchase #2 $127.59   8.381%   $138.2833   $138.28   $138.30   $138.30  
  Purchase #3 $12.40   3.516%   $12.836   $12.84   $12.85   $12.80  
  Purchase #4 $103.33   6.740%   $110.2944   $110.29   $110.30   $110.30  
  Purchase #5 $45.68   9.699%   $50.1105   $50.11   $50.10   $50.10  
  Purchase #6 $49.95   6.193%   $53.0434   $53.04   $53.05   $53.00  
  Purchase #7 $227.47   7.728%   $245.0489   $245.05   $245.05   $245.00  
  7 purchase totals = $594.70       $639.4078   $639.40   $639.45   $639.30