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 $39.62   7.850%   $42.7302   $42.73   $42.75   $42.70  
  Purchase #2 $7.88   5.428%   $8.3077   $8.31   $8.30   $8.30  
  Purchase #3 $35.79   5.718%   $37.8365   $37.84   $37.85   $37.80  
  Purchase #4 $181.40   4.604%   $189.7517   $189.75   $189.75   $189.80  
  Purchase #5 $155.91   13.830%   $177.4724   $177.47   $177.45   $177.50  
  Purchase #6 $73.41   12.644%   $82.692   $82.69   $82.70   $82.70  
  Purchase #7 $196.70   3.711%   $203.9995   $204.00   $204.00   $204.00  
  7 purchase totals = $690.71       $742.7900   $742.79   $742.80   $742.80