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 $9.78   3.170%   $10.09   $10.09   $10.10   $10.10  
  Purchase #2 $107.85   13.266%   $122.1574   $122.16   $122.15   $122.20  
  Purchase #3 $94.41   9.495%   $103.3742   $103.37   $103.35   $103.40  
  Purchase #4 $204.67   6.711%   $218.4054   $218.41   $218.40   $218.40  
  Purchase #5 $67.79   13.754%   $77.1138   $77.11   $77.10   $77.10  
  Purchase #6 $210.67   11.346%   $234.5726   $234.57   $234.55   $234.60  
  Purchase #7 $200.70   11.279%   $223.337   $223.34   $223.35   $223.30  
  7 purchase totals = $895.87       $989.0504   $989.05   $989.00   $989.10