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 $227.95   13.507%   $258.7392   $258.74   $258.75   $258.70  
  Purchase #2 $195.57   4.453%   $204.2787   $204.28   $204.30   $204.30  
  Purchase #3 $122.64   10.706%   $135.7698   $135.77   $135.75   $135.80  
  Purchase #4 $232.27   4.158%   $241.9278   $241.93   $241.95   $241.90  
  Purchase #5 $181.42   5.471%   $191.3455   $191.35   $191.35   $191.30  
  Purchase #6 $135.54   11.147%   $150.6486   $150.65   $150.65   $150.60  
  Purchase #7 $119.32   5.301%   $125.6452   $125.65   $125.65   $125.60  
  7 purchase totals = $1,214.71       $1,308.3548   $1,308.37   $1,308.40   $1,308.20