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 $121.95   11.761%   $136.2925   $136.29   $136.30   $136.30  
  Purchase #2 $220.87   5.627%   $233.2984   $233.30   $233.30   $233.30  
  Purchase #3 $199.47   11.419%   $222.2475   $222.25   $222.25   $222.20  
  Purchase #4 $241.56   6.737%   $257.8339   $257.83   $257.85   $257.80  
  Purchase #5 $161.95   7.590%   $174.242   $174.24   $174.25   $174.20  
  Purchase #6 $49.30   5.860%   $52.189   $52.19   $52.20   $52.20  
  Purchase #7 $13.31   10.221%   $14.6704   $14.67   $14.65   $14.70  
  7 purchase totals = $1,008.41       $1,090.7737   $1,090.77   $1,090.80   $1,090.70