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 $20.70   7.640%   $22.2815   $22.28   $22.30   $22.30  
  Purchase #2 $177.35   7.757%   $191.107   $191.11   $191.10   $191.10  
  Purchase #3 $61.44   10.350%   $67.799   $67.80   $67.80   $67.80  
  Purchase #4 $134.62   7.608%   $144.8619   $144.86   $144.85   $144.90  
  Purchase #5 $102.94   8.672%   $111.867   $111.87   $111.85   $111.90  
  Purchase #6 $110.35   7.471%   $118.5942   $118.59   $118.60   $118.60  
  Purchase #7 $7.35   9.637%   $8.0583   $8.06   $8.05   $8.10  
  7 purchase totals = $614.75       $664.5689   $664.57   $664.55   $664.70