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 $7.67   4.172%   $7.99   $7.99   $8.00   $8.00  
  Purchase #2 $171.70   9.386%   $187.8158   $187.82   $187.80   $187.80  
  Purchase #3 $80.85   8.628%   $87.8257   $87.83   $87.85   $87.80  
  Purchase #4 $197.66   7.424%   $212.3343   $212.33   $212.35   $212.30  
  Purchase #5 $27.92   12.146%   $31.3112   $31.31   $31.30   $31.30  
  Purchase #6 $10.94   11.323%   $12.1787   $12.18   $12.20   $12.20  
  Purchase #7 $141.96   5.570%   $149.8672   $149.87   $149.85   $149.90  
  7 purchase totals = $638.70       $689.3229   $689.33   $689.35   $689.30