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 $123.59   13.203%   $139.9076   $139.91   $139.90   $139.90  
  Purchase #2 $250.85   5.621%   $264.9503   $264.95   $264.95   $265.00  
  Purchase #3 $66.77   12.142%   $74.8772   $74.88   $74.90   $74.90  
  Purchase #4 $18.59   12.647%   $20.9411   $20.94   $20.95   $20.90  
  Purchase #5 $204.65   11.586%   $228.3607   $228.36   $228.35   $228.40  
  Purchase #6 $119.60   13.463%   $135.7017   $135.70   $135.70   $135.70  
  Purchase #7 $210.36   5.174%   $221.244   $221.24   $221.25   $221.20  
  7 purchase totals = $994.41       $1,085.9826   $1,085.98   $1,086.00   $1,086.00