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 $31.57   8.625%   $34.2929   $34.29   $34.30   $34.30  
  Purchase #2 $13.82   8.424%   $14.9842   $14.98   $15.00   $15.00  
  Purchase #3 $53.51   6.445%   $56.9587   $56.96   $56.95   $57.00  
  Purchase #4 $99.30   12.829%   $112.0392   $112.04   $112.05   $112.00  
  Purchase #5 $120.81   10.688%   $133.7222   $133.72   $133.70   $133.70  
  Purchase #6 $137.53   7.109%   $147.307   $147.31   $147.30   $147.30  
  Purchase #7 $30.40   4.457%   $31.7549   $31.75   $31.75   $31.80  
  7 purchase totals = $486.94       $531.0591   $531.05   $531.05   $531.10