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 $133.42   3.695%   $138.3499   $138.35   $138.35   $138.30  
  Purchase #2 $160.69   5.703%   $169.8542   $169.85   $169.85   $169.90  
  Purchase #3 $124.25   4.540%   $129.891   $129.89   $129.90   $129.90  
  Purchase #4 $215.78   12.503%   $242.759   $242.76   $242.75   $242.80  
  Purchase #5 $47.30   12.588%   $53.2541   $53.25   $53.25   $53.30  
  Purchase #6 $23.27   8.557%   $25.2612   $25.26   $25.25   $25.30  
  Purchase #7 $235.57   12.214%   $264.3425   $264.34   $264.35   $264.30  
  7 purchase totals = $940.28       $1,023.7119   $1,023.70   $1,023.70   $1,023.80