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 $17.97   3.373%   $18.5761   $18.58   $18.60   $18.60  
  Purchase #2 $37.87   5.695%   $40.0267   $40.03   $40.05   $40.00  
  Purchase #3 $21.77   4.821%   $22.8195   $22.82   $22.80   $22.80  
  Purchase #4 $58.91   5.699%   $62.2673   $62.27   $62.25   $62.30  
  Purchase #5 $232.58   13.518%   $264.0202   $264.02   $264.00   $264.00  
  Purchase #6 $234.59   7.120%   $251.2928   $251.29   $251.30   $251.30  
  Purchase #7 $131.27   13.794%   $149.3774   $149.38   $149.40   $149.40  
  7 purchase totals = $734.96       $808.3800   $808.39   $808.40   $808.40