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 $247.43   13.865%   $281.7362   $281.74   $281.75   $281.70  
  Purchase #2 $147.42   9.103%   $160.8396   $160.84   $160.85   $160.80  
  Purchase #3 $211.54   5.745%   $223.693   $223.69   $223.70   $223.70  
  Purchase #4 $49.48   6.471%   $52.6819   $52.68   $52.70   $52.70  
  Purchase #5 $8.55   9.262%   $9.3419   $9.34   $9.35   $9.30  
  Purchase #6 $245.51   13.760%   $279.2922   $279.29   $279.30   $279.30  
  Purchase #7 $30.79   11.214%   $34.2428   $34.24   $34.25   $34.20  
  7 purchase totals = $940.72       $1,041.8276   $1,041.82   $1,041.90   $1,041.70