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 $78.52   6.306%   $83.4715   $83.47   $83.45   $83.50  
  Purchase #2 $8.48   3.173%   $8.7491   $8.75   $8.75   $8.70  
  Purchase #3 $59.65   6.588%   $63.5797   $63.58   $63.60   $63.60  
  Purchase #4 $176.33   6.208%   $187.2766   $187.28   $187.30   $187.30  
  Purchase #5 $235.41   9.870%   $258.645   $258.65   $258.65   $258.60  
  Purchase #6 $180.92   7.632%   $194.7278   $194.73   $194.75   $194.70  
  Purchase #7 $215.33   8.556%   $233.7536   $233.75   $233.75   $233.80  
  7 purchase totals = $954.64       $1,030.2033   $1,030.21   $1,030.25   $1,030.20