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 $27.51   10.498%   $30.398   $30.40   $30.40   $30.40  
  Purchase #2 $18.66   12.421%   $20.9778   $20.98   $21.00   $21.00  
  Purchase #3 $123.59   4.460%   $129.1021   $129.10   $129.10   $129.10  
  Purchase #4 $178.42   6.275%   $189.6159   $189.62   $189.60   $189.60  
  Purchase #5 $235.72   6.403%   $250.8132   $250.81   $250.80   $250.80  
  Purchase #6 $14.51   12.712%   $16.3545   $16.35   $16.35   $16.40  
  Purchase #7 $48.56   11.130%   $53.9647   $53.96   $53.95   $54.00  
  7 purchase totals = $646.97       $691.2262   $691.22   $691.20   $691.30