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 $20.58   12.237%   $23.0984   $23.10   $23.10   $23.10  
  Purchase #2 $75.84   12.303%   $85.1706   $85.17   $85.15   $85.20  
  Purchase #3 $113.96   5.139%   $119.8164   $119.82   $119.80   $119.80  
  Purchase #4 $147.29   7.355%   $158.1232   $158.12   $158.10   $158.10  
  Purchase #5 $141.25   4.367%   $147.4184   $147.42   $147.40   $147.40  
  Purchase #6 $236.49   5.403%   $249.2676   $249.27   $249.25   $249.30  
  Purchase #7 $202.74   3.204%   $209.2358   $209.24   $209.25   $209.20  
  7 purchase totals = $938.15       $992.1304   $992.14   $992.05   $992.10