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 $8.60   9.395%   $9.408   $9.41   $9.40   $9.40  
  Purchase #2 $27.70   12.194%   $31.0777   $31.08   $31.10   $31.10  
  Purchase #3 $22.74   12.317%   $25.5409   $25.54   $25.55   $25.50  
  Purchase #4 $134.46   9.658%   $147.4461   $147.45   $147.45   $147.40  
  Purchase #5 $12.89   9.360%   $14.0965   $14.10   $14.10   $14.10  
  Purchase #6 $41.34   12.378%   $46.4571   $46.46   $46.45   $46.50  
  Purchase #7 $86.69   13.170%   $98.1071   $98.11   $98.10   $98.10  
  7 purchase totals = $334.42       $372.1334   $372.15   $372.15   $372.10