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 $238.33   8.652%   $258.9503   $258.95   $258.95   $259.00  
  Purchase #2 $45.52   13.637%   $51.7276   $51.73   $51.75   $51.70  
  Purchase #3 $235.44   9.333%   $257.4136   $257.41   $257.40   $257.40  
  Purchase #4 $184.71   11.622%   $206.177   $206.18   $206.20   $206.20  
  Purchase #5 $219.81   11.487%   $245.0596   $245.06   $245.05   $245.10  
  Purchase #6 $31.43   5.397%   $33.1263   $33.13   $33.15   $33.10  
  Purchase #7 $132.54   11.173%   $147.3487   $147.35   $147.35   $147.30  
  7 purchase totals = $1,087.78       $1,199.8031   $1,199.81   $1,199.85   $1,199.80